The correct answer is the last option.
To credit a source means to ACKNOWLEDGE WHERE YOU FOUND A SOURCE. In order to avoid plagiarism, the writer has to cite the sources where he found information, being the most common citation formats APA or MLA. The citation style to use will depend on the topic to discuss in the paper.
She was kind of mysterious at the beginning but through the play she became less mysterious.
The correct answer is C. Modern Poet.