The correct option is C can't
The correct answer is Option C.
CAN'T expresses inability.
The person in the sentence is not able to express how glad he is.
Thus CAN'T is the correct answer.
The remaining options are incorrect because:
WAS ABLE TO expresses ability and not inability.
COULDN'T is the past tense of CAN'T. The sentence is in the present tense.
HAD TO expresses compulsion.
HAVE is used to express the meaning of possession.