I use x to denote theta.

since (tanx-secx)^2 =(tanx)^2-2tanx*secx+(secx)^2 , A and B are not correct.

Then check C and D, on the bottom, (cosx)^2=1-(sinx)^2 (due to trig identity: (cosx)^2+(sinx)^2=1) So C is not correct.

so D

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