Consider the following array. Which algorithm out of the following options uses the least number of comparisons (among the array elements) to sort the above array in ascending order?

The given array is already sorted in ascending order:

Here, the number of elements is $n = 8$.

Let us analyze the number of comparisons for each algorithm on an already sorted array:

1. Insertion sort: In the best case (when the array is already sorted), each element from index $1$ to $n-1$ is compared only once with its immediate predecessor to determine that it is already in the correct position. The total number of comparisons is:

2. Selection sort: It always scans the remaining unsorted subarray to find the minimum element, regardless of the initial order. It requires:

3. Mergesort: The standard mergesort divides the array recursively and merges sorted subarrays. Even in the best case, it requires $\Theta(n \log_2 n)$ comparisons (at least $12$ comparisons for $n=8$).

4. Quicksort (last element as pivot): Since the array is already sorted and the pivot is chosen as the last element, the pivot is always the largest element in the current partition. This leads to the worst-case unbalanced recursion tree of depth $n$, requiring:

Hence, Insertion sort (Option C) uses the least number of comparisons.