The correct option is D obvious
Covert means not openly acknowledged or displayed. The opposite of covert is obvious, or easily perceived or understood; clear, self-evident, or apparent. Hence, Option C is correct. Among the rest of the options, flimsy means insubstantial and easily damaged, inexplicable means that which cannot be explained, unjustifiable means that which cannot be supported with logic .These do not express this antonym, hence incorrect.