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.

Explanation:
The answer should be “Match a literal + symbol” because there is a backslash symbol before the plus, so it should match a literal +.