Explanation

(d)
In a chemical reaction, coefficient represents mole of that substance.
$X + 2Y \longrightarrow Z$
This indicates 1 mole of $X$ reacts with 2 moles of $Y$ to form 1 mole of$Z$.
So, 5 moles of $X$ will require 10 moles of$Y$. But we have taken only 9 moles of $Y$.
Hence, $Y$ is in limiting quantity. Hence, we determine product from $Y$.
Thus, 5 moles of $X$ react with 9 moles of $Y$ to form 4 moles of $Z$.