@srestha, kindly notice that in question its written two sorted list, not array.
in this case,
best case:- all elements of 2nd list is greater than that of 1st list then minimum comparison is Min(m,n) as it will take only one iteration.for comparison rest will be inserted as it is.
and worst case:- let 2 sorted list be 1,2,3,4 and 1,2,3,4 so here total comparison for merging should be =7(last element need not be compared) so total comparison= m+n-1= O(m+n).