Question

Which of the following sentences contains an infinitive? A. Heather wants to go to the store with us. B. The car was parked next to the church. C. Let's walk to the picnic. D. Dave will take us to the field before the game.

Answer 1

I deleted my earlier reply because I was mistaken. After further reading, here ( http://owl.english.purdue.edu/owl/resource/627/03/), I think the answer is A, and the infinitive is "to go."
I hope you look over the information in the link. It can really help you.

Answer 2

Yes, A is correct. Jenny, do you understand the concept of an infinitive? That is the form of the verb that is not cast in a form (conjugated) to indicate person (who: I, you, he, she, we, they) or tense (time: past, present, future, and so on).

The infinitive is recognizable (usually) by virtue of being the verb stem preceded by the marker "to." Of course, "to" can be used in other ways. In the sentences you have posted, you are meant to distinguish between "to" used as an infinitive marker and "to" used as a preposition ("to the store," for example). If the "to" is followed by a noun or noun phrase, it is a preposition. If it precedes a verb stem, it is part of the infinitive.

I'm sure the link that peeps has directed you to will have even more great information on infinitives.

p.s. I said "usually" above because in combination with some other verbs, the infinitive loses its marker. In sentence A above, note that the infinitive is part of a larger verb phrase: "wants to go." Some verbs -- I can't think of any at the moment -- require you to drop the "to" when the infinitive follows as part of the phrase. Just be aware of that.