SAT Question 8a714fa1: Percentages answer and explanation with Desmos solution

Choice A is correct. Increasing the positive quantity $x$ by $9\%$ is the result of adding $9\%$ of $x$ to $x$. $9\%$ of $x$ can be represented algebraically as $\frac{9}{100}\times x$, or $0.09x$. Adding this expression to $x$ yields $x+0.09x$, or $1.09x$.