If 4 : 15 :: 8 : $x$ is a true proportion, which equation correctly applies the cross-product rule?

In a : b :: c : d the outer terms are the extremes and the inner terms are the means, and extremes × extremes = means × means. Here the extremes are 4 and $x$ and the means are 15 and 8, so $4\times x=15\times 8=120$, giving $x=30$. The last option adds instead of multiplying, which is not the proportion test.