The correct answer is B. ?.
A wildcard is a special character that can be used to represent one or more characters in a file name. The asterisk (*) wildcard can be used to represent any number of characters, while the question mark (?) wildcard can be used to represent any single character.
In the context of the question, the wildcard ? is used to match a single character. For example, the command ls *.txt would list all files in the current directory that end with the extension .txt. The command ls ?.txt would list all files in the current directory that end with a single character and the extension .txt.
The other options are not correct. The asterisk (*) wildcard can be used to represent any number of characters, but it cannot be used to represent a single character. The square brackets [ijk] wildcard can be used to represent any one of the characters i, j, or k, but it cannot be used to represent a single character. The exclamation mark (!) wildcard can be used to negate a character class, but it cannot be used to represent a single character.