Choice A is correct. Let $x$ represent the percentage of $300$ that is $75$. This can be written as \dfrac{x}{100}$$(300) = $75$, ог $3x$ = $75$. Dividing both sides of this equation by $3$ yields $x$=25. Therefore, $25$% of $300$ is $75$

Choice B is incorrect. $50$ % of $300$ is $150$, not $75$.

Choice C is incorrect. $75$ % of $300$ is $225$, not $75$.

Choice D is incorrect. $225$ % of $300$ is $675$, not $75$.