The work done by a force in moving a body is W=F⋅s where, s is the displacement vector. In circular path, the force is centripetal, thus always acting towards the centre along the radial direction. However, the displacement is always along tangent to the circle at the instantaneous position of object. Hence, θ=90∘ Thus, ‌‌W=F⋅s=Fs‌cos‌90∘=0