Single digit positive numbers are: $1$, $2$, $3$, $4$, $5$, $6$, $7$, $8$, and $9$.

To maximize the value of $(p + q) (r + s)$, we need to ensure that we multiply the biggest numbers possible. So, we will only consider the numbers $6$, $7$, $8$, and $9$. Also, we should ensure that the numbers being multiplied are as close in value as possible.

So, the required answer = $(6 + 9) (7 + 8) = 15 \times 15 = 225$

Hence, option (B) is correct.