b
(b) ${(\Delta Q)_P} = \mu {C_P}\Delta T$ and ${(\Delta Q)_V} = \mu {C_V}\Delta T$
==> $\frac{{{{(\Delta Q)}_V}}}{{{{(\Delta Q)}_P}}} = \frac{{{C_V}}}{{{C_P}}} = \frac{{\frac{3}{2}R}}{{\frac{5}{2}R}} = \frac{3}{5}$ $\left[ {\because \,{{({C_V})}_{mono}} = \frac{3}{2}R,\,{{({C_P})}_{mono}} = \frac{5}{2}R} \right]$
==> ${(\Delta Q)_V} = \frac{3}{5} \times {(\Delta Q)_P} = \frac{3}{5} \times 210 = 126\,J$