Explanation

(a) We treat water like a solid. For each atom average energy is $3k_{B}T.$ Water molecule has three atoms, two hydrogen and one oxygen. The total energy of one mole of water is
$U = 3 \times 3k_{B}T \times N_{A} = 9RT\ \ \ \ \left\lbrack \because k_{B} = \frac{R}{N_{A}} \right\rbrack$
$\therefore$ Heat capacity per mole of water is
$C = \frac{\mathrm{\Delta}Q}{\mathrm{\Delta}T} = \frac{\mathrm{\Delta}U}{\mathrm{\Delta}T} = 9R$