1. My hobby is (collecting stamps.)

A. predicate nominative
B. adjective
C. adverb
D. direct object

I think it's D?

2. (Stamp collecting) is fun for children and adults alike.

A. subject
B. predicate nominative
C. adverb
D. direct object

I think it's A?

3. Many collectors try )to acquire stamps in special areas.)

A. subject
B. adjective
C. adverb
D. direct object

I'm not sure, maybe D?

3 answers

1. incorrect (verb is "is" which is a linking verb and doesn't have direct objects following)

2. correct

3. Yes, it's D. The infinitive phrase is serving as the direct object after the verb "try."

so then #1 is A?

Yes. 1 is A.