. . . . . . . . my opinion you should buy the blue shirt.

The correct preposition to use in this sentence is in. The sentence should read: "In my opinion, you should buy the blue shirt." The phrase "in my opinion" is an idiomatic expression used to introduce a personal viewpoint or belief.

The other options are not suitable in this context:
B. "On my opinion" (Incorrect)
C. "At my opinion" (Incorrect)
D. None of the above (Incorrect)