Explanation

A 2×2 system has a unique solution iff the rows are not proportional, i.e. 7/k ≠ k/7, i.e. k² ≠ 49. Among the choices, option (a), k ≠ 7, is the listed necessary condition and is the intended answer. (Strictly, k = −7 also destroys uniqueness, but no option captures both.)