Answer:
a. 0.0486
Explanation:
Given that the mean arrival rate is 15 customers per hour, and interarrival times are exponentially distributed
When the interarrival times are exponential, no of customers arriving per hour is Poisson with mean = 15
The probability that exactly 10 customers will arrive in 1 hour
where X is a Poisson random variable with mean = 15

Thus we find that probability is 0.04861 and round it off to 4 decimals
Prob = 0.0486
Hence option a is right