JEE Main 18 March 2021 Shift 2 Solved Paper

Generally, d-block elements shows magnetic property due to presence of unpaired electrons in its d-block. Mn i.e. manganese shows +2 and +3 oxidation state.
Both having unpaired electrons as shown below
‌25Mn=[Ar]3d54s2
Mn2+(3d54s0) having 5 unpaired electrons in 3d orbital.
Mn+3,(3d44s0) having 4 unpaired electrons in 3d orbital.
SiO2,p-block element compound is commonly believed to be a typical diamagnetic as no unpaired electrons are present.
MgO and Na2O also have unpaired electrons and hence are diamagnetic.
Hence, Mn3O4 show magnetic property.