The correct answer is A. 30 minutes.
The detention period is the time it takes for water to flow through a settling tank. It is calculated by dividing the volume of the tank by the flow rate of water. The settling velocity is the speed at which particles settle in water. It is calculated by dividing the particle size by the density of water.
In this case, the settling velocity is 0.33 m/s and the depth of the tank is 2.4 m. The detention period is therefore:
$T = \frac{V}{Q} = \frac{2.4 \times 0.33}{0.06} = 30$ minutes
Therefore, the correct answer is A. 30 minutes.
Option B is incorrect because it is twice the detention period calculated above. Option C is incorrect because it is more than twice the detention period calculated above. Option D is incorrect because it is more than three times the detention period calculated above.