LLPSI Exercitia Latina - CAP XX - Ex. 4
According to A&G, 409 cibo and lacte should be Ablative of Means. Not quite sure of the function of “quo” though. Is quo a demonstrative adjective which modifies the substantive cibo?
Lac is a sort of of cibus. Does that give a hint as to the role of quo? Don’t forget it’s a question. I am trying not to translate here as that’s the method you are using.
If quo was missing the answer would be yes, but that’s not the question asked.
Completely stumped!
a) Assuming ablative of means is still relevant then quo should translate to something like “by which means” does this food nourish the small infant? or less clunkily Which food nourishes. . . ?
b) but the primary (?) function of Quo would require a verb of motion, i.e. quo vadis? which I can’t see being applicable here.
Is there a clue in your “lac is a sort of cibus”?
This is far above my paygrade!! 
Quid est cibus infantium pavulorum?
You understand quid…cibus?
Apply that understanding to Quo cibo? Try not to translate it. Forget about ablative of means which doesn’t figure in the textbook you are using. Quo cibo is in the ablative because it is the means by which the infant is fed and it goes with the passive verb alitur.
If you still can’t figure it out either someone else can give you a better hint or I can tell you what the answer is.
You will learn more if you figure it out for yourself.
Last hint : think of another word instead of “which”. (an interrogative)
Fingers crossed that is the correct answer.
If not, I think I shall have to ask you for a translation. 
I think you have gone off at a bit of a tangent here. I am sorry if it’s because I wasn’t clear.
I thought you were questioning the meaning of “Quo cibo”?
You asked “Not quite sure of the function of “quo” though. Is quo a demonstrative adjective which modifies the substantive cibo?”
Quo is not a demonstrative but an interrogative.
so:
Quid est cibus…?
what is the food…
Quo cibo …?
By (means of) what food is etc…?
Maybe this would have been simpler face to face. 
This (from Lingva Latina: A Companion to Familia Romana) might make things clearer.
Ablative Case (Agent and Means/Instrument)
Consider the following sentence:Mārcus Iūliam pulsat. Marcus hits Julia.
If we make that sentence passive, we get:
Iūlia pulsātur ā Mārcō. Julia is hit by Marcus.
In the second sentence, Marcus is no longer the grammatical subject, but
he is still the actor, or agent, of the verb. In the passive voice, the name of the person by whom the action is performed, the agent, is in the ablative preceded by ab or ā (ā Mārcō). This construction is called the ablative of personal agent, that is, when the agent is a person, not a thing or an animal:
Iūlius ab Ursō et Dāvō portātur. Julius is (being) carried by Ursus and Davus. (l.62)
Saccī quī ā Syrō et Lēandrō The bags which are being carried portantur magnī sunt. by Syrus and Leander are big.
(ll.65–66) Dominus ā servō malō timētur. (ll.73–74)
Verba Mēdī ā Lydiā laetā audiuntur. (l.95)When the action is performed by something other than a person—an animal or an inanimate object—the source of the action is expressed by the simple ablative without the preposition ab/ā. The simple ablative here indicates means or cause. This construction, called the ablative of means (also ablative of instrument—Latin ablātīvus īnstrūmentī) is very common both in passive and active sentences: e.g.,
Cornēlius equō vehitur. Cornelius is being transported by a horse. (or, more idiomatically, “he is riding a horse”) (ll.68–69)
Iūlius lectīcā vehitur. Julius is being carried in a litter chair. (l.69)Lȳdia verbīs Mēdī dēlectātur. Lydia is delighted by Medus’s
words. (l.91)Dominus servum baculō verberat. Servī saccōs umerīs portant.
Mēdus viā Latīnā Rōmam ambulat.Sometimes, the agent/means is left unexpressed, e.g.:
Dominī vehuntur. Masters are carried (or “travel”). (l.70)In the sentence Mēdus Lydiam amat et ab eā amātur (ll.78–79), both active and passive are used.
Sorry I went off on a tangent.
Yep, got it - Quo cibo and quid est cibus mean the same. Will have a look at the Companion to LLPSI Familia Romana.
Iterum, multas gratias tibi ago.
The essential difference between “quid cibus” and “quo cibo” in this context is that the former can form a question with an active verb and the latter with a passive verb. What is the food you give to the child? and With what food is the child fed?