SAT Question 312ba47c: Ratios Rates Proportional Relationships And Units answer and explanation with Desmos solution

Choice A is correct. It's given that the ratio of black pens to red pens is $8$ to $1$. Therefore, there are $\frac{1}{8}$ as many red pens as black pens in the box. It's also given that there are $40$ black pens in the box. Therefore, the number of red pens is $\frac{1}{8}$ of the $40$ black pens. Thus, the number of red pens is $40\left(\frac{1}{8}\right)$, or $5$.