To simplify problems like this, we simply need to add our like-terms. Like-terms are those that have the same variables an exponents. In this case, our like-terms are "4x²y" and "6x²y" (note that there can be more than 1 pair in a problem, but this set only has one). We can simply add these together, giving us "10x²y." No other terms can be combined, so our final equation is 10x²y+12z²-2z.
the answer is D because you are combining like terms