@Vikas123 bro read the question properly it is asking about merge procedure not merge sort. you know that merge sort contain 3 functions inside it .
mergesort(a,p,r)
{
{if (p<=r)
q=p+r/2;
mergesort(a,p,q);
mergesort(a,q+1,r);
merge(a,p,q,r);// question is specifically asking about tc of this part
{
}
hope this helps