(D) m+n-1 ..
Merge sort algorithm uses Divide and Conquer.
Hence, we divide both our lists into m and n single-element lists respectively...
Now, we know that first m lists are sorted and after these m lists, the n lists are also sorted...
Let’s call these single element lists as m1, m2 and so on, and n1, n2 and so on...
To merge these lists into one single-sorted list, we compare ..
## m1 with n1.
### Either m2 with n1 or m1 with n2. Here, for sorting 3 elements, 2 comparisons are needed....
#### Observe that, each element in subset of m needs to be compared with each element in subset of n...
We don’t need to compare m1 with other m lists because they are sorted and same goes for n lists…
##### We repeat the process until we have got a sorted list with m + n elements.
This happens when we have made m + n – 1 comparisons...