For this case we can translate the sentence given as:
__________ milk is put on custard?
They put 14 ounces of condensed milk and 12 ounces of evaporated milk.
We must complete the sentence with the most appropriate option. It is noted that the question refers to the amount of milk needed for flan. "Milk" is the feminine and singular subject.
So, the most appropriate option is "Cuánta"
ANswer:
¿Cuánta leche se pone en flan?
Se ponen 14 onzas de leche condensada y 12 onzas de leche evaporada.
