The correct answer is C. Secret
Explanation:
The word covert when used as a noun means a place or area covered by trees that can be used as a shelter or a place to hide, however when covert is used as an adjective means something that is concealed and is a synonym of hidden, clandestine, undercover or secret especially when it refers to an operation, strategy or plan. Thus, in the case of the sentence Antony and Octavius make covert plans to stay in power it means their plans are hidden or secret plans.
