There is a complete sentence before the blank and a continuing idea after it. Therefore, we need a word, or phrase, that bridges the sentence to the next part of the idea.
1. It is not (A) because there is no connecting word after the adjective.
2. It is not (B) because it is an isolated adjective.
3. It is not (C) because it is similar to (A).
4. It is (D).