∵ Given, Molar heat capacity for oxygen =20.8Jmol−1K−1 Q=nC∆T where, Q= value of ∆U in J n= number of moles (=1) C= heat capacity ∆T= temperature Q=1×20.8×60 Thus, ∆T=(−)20 to (+)40=60∘C Hence, for one mole of oxygen, value of ∆U=60×20.8=1248J Hence, option (d) is the correct answer.