The idea is a two part which is separated by a middle comma. After the comma there is a comparative idea. The first part is missing the subject and verb. Therefore, there is also a connector or initial comparative missing.
1. It is not (A) because the structure is not balanced with the one after the comma.
2. It is not (B) because it has no comparative form.
3. It is not (C) because it has no comparative form.
4. It is (D).