B, the coefficient of thermochemical equation represents the quantity of matter, not the quantity of molecules, so b is wrong;

C, thermochemical equation? h2(g)+i2(g)═2hi(g)； △H=+52kJ？ Mol- 1 means that 1molH2(g) and 1molI2(g) are completely reacted.

2mol of HI gas needs to absorb 52kJ of heat, so c is correct;

D, thermochemical equation? h2(g)+i2(g)═2hi(g)； △H=+52kJ？ In mol- 1, the enthalpy change value is positive, indicating that the reaction is endothermic and will not give off heat, so d is wrong.

So choose C.