C. 15a²b², -15a²b²

Why?

 - Well, similar terms looks for the identical variables and it does not include the coefficients. Letter C is the only one with the exact unknown factors.

• A does have the same variable but the second term has exponents and it changes the whole thing, they are no longer the same.
• B has the same coefficients but as explained, variables is what's needed to be identical.
• D is almost like A, if you change the position of the exponent it can create a huge difference.