Important for comprehension. it is the first step to understanding

a. paraphrasing
b. re-reading
c. retention
d. repetition

C. Retention is important for comprehension. It is the first step to understanding.
The word retention refers to the act of retaining something - in this case, knowledge. So, you read something, and then you retain that information in your mind. Over time, you will begin to understand it more and more, and thus it will stay in your mind forever.

Conclusion if that's an answer.