Which letters from the table represents like terms

B and C.

When multiplying binomials, you use a form of the distributive property. With how your lesson has it here, it formed a multiplication table instead.

If you were to complete the table with the actual values, you'd find that:
A=-8x^2
B= 12x
C= -2x
D= 3

As you can see, B and C are the only like terms, or in other words, they can be added together since they have similar variables