The correct option is C 4RT
Initial internal energy,
Ui=nfRT2=20×5×RT2=50RT
[for diatomic gas, f=5]
Final internal energy,
Uf=n1f1RT2+n2f2RT2
⇒Uf=12×5×RT2+2×8×3×RT2=54RT
[if one mole of diatomic gas dissociates, then two moles of monatomic gas are produced, and for monoatomic gas, f=3]
Now, change in internal energy will be,
ΔU=Uf−Ui=54RT−50RT=4RT
Since there is no volume change associated with the process, work done W=0
Hence, heat supplied Q=ΔU=4RT
[from first law of thermodynamics]