Discussion

Explanation:

Enemies of every pair are the pairs formed with all numbers other than the two in the member itself.

∴ If there are n elements then each member has

$$\begin{gathered} {}^{(n-2)}{C}_2=\frac{(n-2)(n-3)(n-4)!}{2(n-4)!} \\ =\frac{1}{2}(n^2-5n+6) \mathrm{enemies} \end{gathered}$$

Hence, option (d).

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