why not can we take λ = 20 and then compute P(1)+P(2)+P(3) , which will give the probability of getting 1 or 3 or 5 request per hour and then take 3/4th of it.
Answer b is right only if we take λ = 15 and then compute P(1)+P(2)+P(3), which will be in requests per 45 mins.