The correct answer is: B. A particle to travel from top surface to bottom sludge zone.
The detention time (t) of a settling tank is the average time that a particle spends in the tank. It is calculated by dividing the volume of the tank by the flow rate of the sewage. The detention time is important because it determines how much time the particles have to settle out of the sewage. The longer the detention time, the more particles will settle out.
Option A is incorrect because it is the time required for a particle to travel along its length, not the time required for it to travel from top surface to bottom sludge zone.
Option C is incorrect because it is the time required for the flow of sewage to fill the tank, not the time required for a particle to travel from top surface to bottom sludge zone.
Option D is incorrect because it is not a valid option.