Answer
If there are six candidates, 15 comparisons must be made.
Work Step by Step
Suppose that the pairwise comparison method is used to determine the winner in an election. In an election with $n$ candidates, the number of comparisons, C, that must be made is determined by the formula $C = \frac{n~(n-1)}{2}$
We can find how many comparisons must be made if there are six candidates. In this case, $n=6$.
$C = \frac{n~(n-1)}{2}$
$C = \frac{6~(6-1)}{2}$
$C = 15$
If there are six candidates, 15 comparisons must be made.