Word: if you want to find a single character using a wildcard, which character would you use? a.** b. * c. ? ? d. ?

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.