There are two ways to figure out the right answer. First, in both cases, you eliminate three answers that are clearly incorrect:
If the property is not used for hunting during A's lifetime, A can pass it to A's heirs, so it is not a life estate.
There is no life estate prior to A's interest, so it is not a remainder interest.
The existence of one or more conditions means that it is not a fee simple absolute.
Then, there are two possible ways (shown in bold) to determine whether (a) or (b) is correct:
All fee simple estates subject to conditions are defeasible fee simple estates.
A fee simple subject to condition subsequent, is one that gives the grantor the option to revoke the grant if the condition occurs, but is not self-executing. This conveyance, in contrast, is self-executing. If there is hunting, then the condition takes effect, without any affirmative action on the party of the grantor. So, it is not a fee simple subject to condition subsequent.
It could have been a fee simple determinable, but in that estate, the property must revert to the owner by operation of law if the condition occurs, while the first condition in this case actually causes the property to go to B and not to O.
The correct subtype of defeasible fee simple estate in this case is called a fee simple subject to executory limitation, but since a fee simple subject to executory limitation is a subtype of defeasible fee simple estate, answer (b) is correct.
To the extent that it is a trick question, it is a trick question because four of the options (a), (c), (d) and (e) are possibilities at one level of specificity, and (b) is a possibility at a more general level of specificity.
If the answer were that it was a fee simple subject to condition subsequent, then both (a) and (b) would be true, because (a) is a subtype of (b), which isn't possible in this kind of multiple choice format. So, (a) cannot be true.