Correct answer is (c) Rs || 1/gm
For explanation: The input impedance can be calculated by performing a small signal analysis at the input node. We need to set Vg and Vdd to 0V and apply a small input voltage at the source. Now, the impedance looking into the source is 1/gm and Rs is parallel to this impedance. Hence, the total input impedance is Rs || 1/gm.