to empty the pool, you need to lower the water level by 150cm. since you can only lower it by 30cm every 2 hours (also same as 15 cm per hour), that means it will take you 10 hours.
150cm*(2 hours)/(30cm) = 150*(1/15) = 150/15 = 10.
6 a.m. + 10 hours = 4 p.m.
So the answer is C.) 4 p.m.