The unit of the spring constant is-

Correct answer: c Jm⁻²

Right. k = F/x with the unit Nm⁻¹, and since J = N·m, Jm⁻² = Nm⁻¹ (also kg s⁻²).

Why the other options are wrong

• a Nm⁻²: Nm⁻² is the unit of stress (force ÷ area). The spring constant is force ÷ length, so Nm⁻¹.
• b Jkg⁻¹: Jkg⁻¹ is energy per kg, as in specific latent heat; it does not match the unit of the spring constant.
• d Nm²kg⁻²: Nm²kg⁻² is the unit of the gravitational constant G, not of the spring constant.