Which of the following terms are like terms?
a) 9x and 9y b)12ac^2 and ac^2 c)8xy and 8xz d)ab^2 and x^2y

Like terms means terms that have the same variable (and with the same exponent). 

A) 9x and 9y <-- those are different variables. This is incorrect.

B) 12ac^2 and ac^2 <-- those are the same variables and same exponent. Correct.

C) 8xy and 8xz <-- different variables, incorrect.

D) ab^2 and x^2y <-- different variables, incorrect.

Correct answer is 

Like terms are terms that have the same variables and also have the same exponents on those variables.

Out of all your choices, answer choice B) 12ac^2 and ac^2 fits that definition.

B is your answer.