Here $PR$ is parallel to $x-$axis so $y-$ coordinate of vertex $P$ and $R$ would always be the same.
For ex- If $P= (-1, \textbf{7})$ then $R=( 2, \textbf{7})$ or $(5, \textbf{7})$
So total number of possible $x-$ coordinate of $P$ and $R$ for a particular $y$ $( -4\leq x \leq 5) = {}^{10}P_2 = 90.$
Similarly, for $11$ different $y-$ coordinates, total coordinates of $P$ and $R = 11 \times 90 = 990.$
Now since, its a right angle at $P$ so vertex $Q$ has same $x$-coordinate as $P.$ So total possible coordinates for vertex $Q ( 6\leq y\leq 16) = {}^{10}C_1= 10.$
For ex- If $P= (\textbf{-1}, 7) $ then $Q= ( \textbf{-1}, 8)$ or $( \textbf{-1}, 16)$ but not $(\textbf{-1}, 7).$
Total number of triangles $= 10 * 990 = 9900$
Ans- C