What does the + symbol mean in the following grep regular expression:

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.

Answer: A