Clemmensen reaction is the conversion of the carbonyl group to CH₂. It is basically a reduction reaction.

The reaction takes place in the presence of the Zinc amalgam and HCl.

Thus, acetone gets converted into an alkane in Clemmensen reduction.

Thus, Cyclohexanone gets converted into cyclohexane and Benzophenone gets converted into diphenylmethane.

The other two options (i) and (iii) do not undergo Clemmensen reduction as they are not ketones.