The correct answer is A. X.
The missing character is the only one that does not appear in the other three options. The other three options all appear in the following sequence:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
The missing character, X, is the only one that does not appear in this sequence.
Option A: X is the only letter that does not appear in the sequence A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.
Option B: Z is the last letter in the sequence A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.
Option C: G is the seventh letter in the sequence A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.
Option D: Q is the seventeenth letter in the sequence A B C D E F G H I J K L M N O P Q R S T U V W X Y Z.