The correctly punctuated sentence is C, that is, "one day Nick's roommate brought home a ferret; he'd seen it at a pet store and couldn't resist it". When a semicolon is used to link two separate ideas, these ideas are given the same degree of importance, they are considered to be equals.
A and B are grammatically incorrect and D is wrongly punctuated. In A, the subordinate clause does not have a subject (it should be "agent Orange, which is believed to be..."). The same happens in B (Johnny Cash, who auditioned for a record producer signing mainly gospel tunes, was told...). In D, there should be a colon after the noun "fetal alcohol syndrome".
