The answer is B.
We use the spring constant in Hooke's Law, which is : F = -kx
It can be rearranged so it is equal to the constant :
k = F ÷ x
Common units of Force (F) : N, kg m/s²
Common units of displacement (x) : cm, m, km
The dimensionally correct unit from the given options is :
⇒ N/km
