The leader might have had some covert reason for the change of his political affiliations.

Answer: Option C

Covert : not openly acknowledged or displayed.
Flimsy : insubstantial and easily damaged.
Inexplicable : unable to be explained or accounted for.
Obvious : easily perceived or understood; clear, self-evident, or apparent.
Unjustifiable : not able to be shown to be right or reasonable.

Antonym of Covert is Obvious