So, what they are looking for is a pronoun instead of the object.
So for Number 1, instead of rewriting the sentence as your answer like:
No, no compré la comida en Walmart.
You would use the pronoun "la" in place of the conjugated form of comprar.
i.e. No, no la compré
Knowing that, I think the rest become much simpler as long as you remember the gender of the nouns.
For number nine make sure you really pay attention to what the direct object is, it´s not as obvious as the others. For the last one, you could use either direct object pronoun as they would correctly answer the question.
Remember if there are two verbs in the sentence the direct object pronoun goes after the second verb
i.e. Voy a llamarla.