The answer is letter C. No, because squaring a binomial always produces a trinomial product.
Not A, it can be binomial or trinomial.
Not B, the product rule wasn't correctly applied.
Not D, the answer must be 4x² - 12x + 9
Let's have another example..
(3x + 9)² =
9x^2 + 54x + 81
Sorry but I can't really explain why C, but for me, C is the correct answer. Not just because the rest are really wrong but there was really an explanation why.
But just in case I'm really wrong, please, kindly correct me. ^-^