An object of mass m is lied to a string of length l and a variable horizontal force is applied on it which is initially zero and gradually increases until the string makes an angle $\theta$ with the vertical. Work done by the force F is

Question

$$(a)\;mg(1-\sin \theta) l \quad (b)\;mgl \quad (c)\; mgl(1+\cos \theta) \quad (d)\;mgl (1-\cos \theta)$$