A is the grammtically correct sentence.
In A, shortgun is the subject, suggested is the verb, and an abundance of possibilites is the direct object.
In B the verb is in passive voice. The subject receives the action. Correct form for a passive sentence would be...
An abundance of possibilities is suggested by the shortgun.
In C the verb is also passive. Correct is..
An abundance of possibilities was suggested by the shortgun.
D is the same as A; it would be correct if "shortgun" were not mispelled.