F = kx

k = ?

F/x

x/F

F + x

F - x

The option B is correct option which is F/x.

Explanation:

Equation: F = kx

By dividing both sides of equations with x:

F/x = kx/x

F/x = k

Hence proved that k = F/x.

Option A F/x is the correct answer

Explanation:

The force when a spring of spring resistance k is elongated by a distance x is given by

F = kx

So we have expression for spring constant, k = F / x

Option A is F/x, which is correct

Option B is x/F which is not correct

Option C is F+x which is not correct

Option D is F-x which is not correct

So Option A F/x is the correct answer.