In the sentence "le bebi yo," the word "la" substitutes for "la leche."
In Spanish, personal pronouns can be used to substitute for nouns to avoid repetition. "Le" seems to be a mistake here because it typically is not used with the verb "beber" in the sense you might expect. Instead, if we stick to the options given and look for a noun that would likely be replaced by "la" in a sentence regarding consumption, "la leche" (the milk) fits best.
So the answer is la leche (though "le" being used in the sentence appears to lead to some confusion, as "el" should have ideally been used).