The answer would be; D. ?
A wildcard is a character that can have a special meaning when using a search engine. One of the functions of wildcard is replacing an unknown character.
The character "?" used for one single character and "*" is used for any amount of character. The "??" symbol would replace 2 unknown characters. But the symbol "*" and "**" wouldn't have any differences since they both will represent any number of unknown character.