The correct option is C Hg
This cell is also called mercury cathode cell. In this cell, Hg flows along the bottom of the cell and acts as cathode in the left hand compartment fitted with graphite anodes in brine (NaCl) solution, whereas the right hand compartment is fitted with Fe cathodes dipped in very dilute NaOH solution and Hg acts as anode in this compartment. Thus, Hg acts as an intermediate electrode by induction, cathode on LHS and anode in RHS.