In this sentence, the verb form is given in the bracket and we need to fill the blank with its adjective form. Achieve basically means to successfully bring about or reach (a desired objective or result) by effort, skill, or courage. The adjective form of it would describe something able to be brought about or reached successfully.
Option A: Is a noun form of the same word, meaning a thing that is done successfully with effort, skill, or courage.
Option B: Achievable is an adjective form which describes that can be reached successfully. It is the correct answer.
Option C: Being in the possibility of achieving something
Option D: Is a wrong word.
Option B is the correct answer.