Choose the correct "part of speech" for the following sentence: 

"My father _____ fly to Australia. I am not sure, though".

The correct option is (C) because Modal auxiliary verbs (can, could, may, might, must, shall, should, will, would) show a variety of meanings. They represent ability, permission, necessity, and degree of certainty. These are always followed by the simple form of the verb. "Might" is used as the past tense of may, to express hesitant permission, past possibility, remote present possibility, purpose in the past, etc.