Rashid and Derek sent each other many e-mails over the summer.
There are only two reciprocal pronouns "each other" and " one another". This means option B is incorrect because it does not have a reciprocal pronoun. Reciprocal pronouns indicate that two or more people are reciprocating their among themselves. In the correct sentence, the emails are going back and forth between Rashid and Derek. The emails are being reciprocated because they are both emailing each other.