Answer:
a
b
Step-by-step explanation:
From the question we are told that
The mean is 
The standard deviation is 
Generally 2 year is equal to 24 months
Generally the percentage of total production will the company expect to replace is mathematically represented as

Generally 

Generally from the z-table

So

Converting to percentage

=>
Generally the duration that should be the guarantee period if Accrotime does not want to make refunds on more than 6% is mathematically evaluated as

=> 
From the normal distribution table the z-score for 0.06 at the lower tail is

So

=>