The result of increasing the quantity x by 400 % is 60 . What is the

Choice A is correct. It's given that the result of increasing the quantity $x$ by $400\%$ is $60$. This can be written as $x+\left(\frac{400}{100}\right)x=60$, which is equivalent to $x+4x=60$, or $5x=60$. Dividing each side of this equation by $5$ yields $x=12$. Therefore, the value of $x$ is $12$.

Choice B is incorrect. The result of increasing the quantity $15$ by $400\%$ is $75$, not $60$.

Choice C is incorrect. The result of increasing the quantity $240$ by $400\%$ is $\mathrm{1,200}$, not $60$.

Choice D is incorrect. The result of increasing the quantity $340$ by $400\%$ is $\mathrm{1,700}$, not $60$.