A. NaCl is diamagnetic because all electrons are paired in Na+ and in Cl− . So, it shows diamagnetism.
B. Fe3O4 is ferrimagnetic because of the presence of the unequal alignment of magnetic moment in opposite direction.
C. O2 molecule has two unpaired electrons. So, it is paramagnetic.
D. In MnO, the orientation of the electrons of Mn and O is such that they cancel their effects, hence it is antiferromagnetic.