Correct option is C
Work done by p in one hour = 1/4
Work done by Q and R in one hour = 1/3
Work done by P and R in one hour = 1/2
Work done by R in one hour = 1/2- 1/4= 1/4
Work done by Q in one hour = 1/3- 1/4= 1/12
Therefore, time taken by Q to complete the work = 12 hours.