Explanation

(d)
Given, $R = \left\{ (x,\ y):4x + 3y = 20 \right\}.$
Since, $R$ is a relation on $N$, therefore $x,\ y$ are the elements of $N$. But in options (a) and (b) elements are not natural numbers and option (c) does not satisfy the given relation $4x + 3y = 20$.