What does the ?symbol mean in the following grep regular expression:
grep ‘^d[aei]\\?d$’ /usr/share/dict/words

A.
Match the preceding character set ([aei]) one or more times.

B.
Match the preceding character set ([aei]) zero or more times.

C.
Match the preceding character set ([aei]) zero or one times.

D.
Match a literal ?symbol.