Equivalent expressions SAT question

Choice D is correct. The given expression shows addition of two like terms. Therefore, the given expression is equivalent to $(50+5)x^2$, or $55x^2$.

Choice A is incorrect. This expression is equivalent to $(50)(5)x^2$, not $(50+5)x^2$.

Choice B is incorrect. This expression is equivalent to $(\frac{50}{5})x^2$, not $(50+5)x^2$.

Choice C is incorrect. This expression is equivalent to $(50-5)x^2$, not $(50+5)x^2$.