sat-dog

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

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

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

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