SAT Question a478f9f5: Probability And Conditional Probability answer and explanation with Desmos solution

Choice A is correct. If one of the gemstones is selected at random, the probability of selecting a gemstone of color Y is equal to the number of gemstones of color Y divided by the total number of gemstones. According to the table, there are $3$ gemstones of color Y, and it's given that the total number of gemstones is $157$. Therefore, if one of the gemstones is selected at random, the probability of selecting a gemstone of color Y is $\frac{3}{157}$.