The "bent" you are thinking of is an adjective, so its opposite cannot
be any of those nouns. Its opposite would be "straight". You need
a definition of "bent" as a
noun.
bent: An individual tendency, disposition, or inclination.
The natural bent of my mind was to science. (Thomas Paine)
ineptitude: awkwardness, clumsiness, incompetence
You would not believe the ineptitude of the carpenter we hired.
Your bent is (approximately) your competence; your ineptitude is (approximately) your incompetence.
Not an exact antonym pair in my opinion, but the best we can do given those choices!
CJ