Option A consists of two independent clauses, therefore there's a coordinating conjunction in between them to help the sentence flow.
<span>d. inconstancy is the answer</span>
Which one of what? You don't say the choices:)
This is true. The general theory regarding personality development is that some aspects of our personality remain constant, while others will change due to environment. This entails a mixture of "nurture and nature" with regard to personality.