A is correct.
Don’t be fooled by the keyword "thus" thrown into the middle of the argument. Although "thus" often signifies a conclusion, in this case it introduces an intermediate conclusion, which can be thought of as a second premise. We know that some people like green carnations around St. Patrick’s Day, and green carnations are hard to breed. White carnations, on the other hand, are cheap and easy to dye. The florist’s final conclusion- that it is wise for florists to stock up on white carnations in anticipation of St. Patrick’s day- follows these supporting premises.
Remember that keywords such as "thus," "therefore," or "nevertheless" can help you identify conclusions, but don’t offer the final word on argument structure.
(A) exactly matches our conclusion. Keep it.
(B) is a premise. Throw it out.
(C) is yet another premise- eliminate it.
(D) might seem tempting, since the florist talks last about how white carnations are inexpensive and easy to dye green. However, remember that conclusions and premises can come anywhere in an argument. Just because something is stated last doesn’t mean it is a conclusion.
(E) offers another premise, like (B) and (C). Eliminate this answer choice.