(a) and (b) are in fact the same statements and both are correct, N has slightly greater ionisation energy than oxygen which is against periodic trend. This exception is due to completely half-filled (2p3) orbital in nitrogen that makes ionisation slightly difficult than oxygen.(c) Also correct: Although N has greater first ionisation potential than oxygen, two values of ionisation potentials are comparable since they are adjacent in a period, i.e. electrons are removed from same orbit during ionisation.(d) Incorrect - opposite to (c). of the bonded atoms which in turn has periodic trend in long form of periodic table.