Assertion (A) : According to molecular orbital theory, $O_2$ molecule has paramagnetic in nature. 
Reason(R)   : No unpaired electrons in anti-bonding orbital’s $({\mathrm{\pi }}^{*}2{{\mathrm{p}}_{\mathrm{x}}}^0={\mathrm{\pi }}^{*}2{{\mathrm{p}}_{\mathrm{y}}}^0)$  of  $O_2$ molecule.