Can I say "Which area of triangle a or triangle b is larger"?

I am a math teacher in Asia. In one math question there are two triangles, A and B. I want to ask which has the larger area. Should I ask:

"Which area of triangle a or triangle b is larger"?

or

"Which triangle, a or b, has a larger area"?

The second one would be more correct: "Which triangle, A or B, has a larger area?"

However, a better way (in my opinion) to phrase it would be simply "Which triangle has a larger area?", if there are only the triangles A and B to choose from.

The second choice you present is correct as you present it. The first choice would be improved if you eliminate the "of" and enclose "triangle a or trianble b" with commas. The "of" in the first choice as you present it implies that triangle a has more than one area.