a
Work done by the system in the isothermal process
$DA\,is\,W = 2.303nRT\,{\log _{10}}\frac{{{P_D}}}{{{P_A}}}$
$ = 2.303 \times 2\,R \times 300\,{\log _{10}}\frac{{1 \times {{10}^5}}}{{2 \times {{10}^5}}} = - 414R$
Therefore work done on the gas is $+414\,R.$