Hi there.
1. Incorrect
"Voy a las" means that he will go to the and "comprar" means to buy.
If you put both together, it doesn't make much sense so that tells you its grammatically incorrect. We can assume by the given sentence that hes going to a store to buy something. The word 'store' is what hes missing here. For this to be correct, you would say; Voy a la tiendo para comprar algo.
2. Correct
The sentence given means that Juan is going to give someone something. It makes sense so nothing would need to be added to the sentence making is grammatically correct.
3. Nos
"Llaman del hospital" means that the call the hospital. We essentially need to find a word that fits the blank and makes sense. If we use te, me, and lo, they don't really fit and don't really fit the blank given in the sense that they are trying to call the hospital, not for the hospital to call them. Nos is the only word that makes sense here.
4. Before
Some spanish direct object pronouns would be nos, los, las, and os. If we used all of them before the verb, the one that makes the most sense is los. Thus, this means that the answer is before.
5. No
The correct sentence is "I will cook it". Translating it would be lo cocinare. As you can tell theres no "a" in the translated sentence making the answer "no".
Best of Luck!