$P_{N_2} + P_{H_2O} = 1 atm$
$\therefore P_{H_2O} = 0.3\;atm$
$\therefore P_{N_2}$ = 0.7 atm in 1 vessel .
Now new pressure of $N_2$ in another vessel may be calculated as
$P_1V_1 = P_2V_2$
$\therefore 0.7 \times V_1 = P_2\times \large\frac{V_1}{3}$atm
$\therefore P_{N_2}$ = 2.1 atm
$\Rightarrow P_{H_2O} = 0.3$atm
$P_T = 2.4$ atm
$P_{H_2O}$ remains constant at constant temperature
Hence answer is (D)