Choice B is the correct answer. Check if this is a Perfect Square Trinomial$(a+b)^2 = a^2 + 2ab + b^2$.

1. First term is $x^2$.
2. Last term is $36 = 6^2$.
3. Middle term is $12x$. Does $2(x)(6) = 12x$? Yes.

Therefore, it factors to $(x + 6)^2$.

Choice A is incorrect because this is the factorization for $x^2 - 36$ (Difference of Squares). Choice C is incorrect because the middle term would be $15x$. Choice D is incorrect because the middle term would be $13x$.