Which one of the following is the tightest upper bound that represents the number of swaps required to sort $n$ numbers using selection sort?

In selection sort, the algorithm divides the input list into two parts: a sorted sublist and an unsorted sublist. In each of the $n-1$ passes, it finds the minimum element from the unsorted sublist and places it at the beginning of the unsorted sublist using at most one swap.

Therefore, the maximum number of swaps performed by selection sort is $n - 1$, which is bounded by $O(n)$.

Thus, the tightest upper bound representing the number of swaps is $O(n)$ (Option B).