What is one concept that makes a person who they are?

personality

Explanation:

-the combination of characteristics or qualities that form an individual's distinctive character.

"she had a sunny personality that was very engaging"

Hope this helped ^^

Self-concept is how you perceive your behavior, abilities, and unique characteristics. 1 For example, beliefs such as "I am a good friend" or "I am a kind person" are part of an overall self-concept. ... At its most basic, self-concept is a collection of beliefs one holds about oneself and the responses of others.

a.) it provides a clue to solve the mystery.

quite an interesting way to teach topic sentences, because the answer to this question is the best definition of a topic sentence that one can present, it is as if answering the questions taught you how to do it, this is some real teaching, it is terribly well-done.

the topic sentence in this paragraph is:

topic sentences writers stay on track and keep a paragraph from becoming an aimless collection of sentences about an idea.

which incidentally tells you what a topic sentence should be.