21) The answer is (C)

Consider the six squares of side one formed by the points $(x, y)$ such that $i \leq x \leq i+1$ and that $j \leq y \leq j+1$, with $i=0,1$ and $j=0,1$, 2. The intersection of each of these squares with $D$ is the triangle lying in the upper left part relative to the diagonal that connects the points $(i, j)$ and $(i+1, j+1)$, so its area is equal to $\frac{1}{2}$. Multiplying by 6 gives that the area of $D$ is equal to 3.
21) Let $D$ be the domain in the Cartesian plane consisting of points $(x, y)$ such that $x-[x] \leq$ $y-[y], 0 \leq x \leq 2,0 \leq y \leq 3$ (recall that $[a]$ denotes the integer part of $a$, i.e., the greatest integer less than or equal to $a$). The area of $D$ is
(A) 1
(B) 2
(C) 3
(D) 4
(E) 6.