so when we add Q amount of heat 3 moles of diaatomic breaks to monoatomic...so we have 6 moles monoatomic gas....so total dU= finalU- initial U....... = (6*3/2 RT + 2*5/2 RT ) - 5*5/2 RT = 14RT - 12.5 RT = 1.5 RT....
so the answer is b, 1.5 RT.... got it...it's a simple problem..