If the time of concentration is 9 minutes, then the intensity of rainfall according to British Ministry of Health formula will be A. 4 mm/hr B. 10 mm/hr C. 20 mm/hr D. 40 mm/hr

The correct answer is $\boxed{\text{C}}$, 20 mm/hr.

The British Ministry of Health formula for the intensity of rainfall is:

$$I = \dfrac{20}{\sqrt{t}}$$

where $I$ is the intensity of rainfall in mm/hr and $t$ is the time of concentration in minutes.

In this case, $t = 9$ minutes, so the intensity of rainfall is:

$$I = \dfrac{20}{\sqrt{9}} = 20 \sqrt{\dfrac{1}{3}} = \boxed{20 \text{ mm/hr}}$$

The other options are incorrect because they do not correspond to the British Ministry of Health formula.