Right option is (b) T(n) <= T(n/4) + T(3n/4) + cn
The explanation is: If there are n/4 elements in one sub-array then T(n/4) comparisons are needed for this sub-array. And T(3n/4) comparison are required for the rest 4n/5 elements, and cn is time required for finding the pivot. If there are more than n/4 elements in one sub-array then other sub-array will have less than 3n/4 elements and time complexity will be less than T(n/4) + T(3n/4) + cn.