Lets us check each option
- a) if there are 2 replications, probability of failing both at same time = .25*.25 = .0625 =6.25%
Availiability = 100-6.25=93% //Not the answer
- b) In case of 4 replications probability of failing all at same time =(0.25) ^{4} =0.0039=0.39%
Availiability = 100-0.39 ~ 99% //Hence the answer
No need to check furthur as all options are more than 4
Answer is B