Paramagnetism is exibited by ?

this option is correct because , in case of homoneuclear molecule it is easy to calculate the magnetic character through molecular orbital theory . but in case of hetronuclear molecule one  easy method is there to find out the magnetic character . magnetic character in case of hetronuclear molecules . 1. count  the total number of valence electrons if it comes out to be even then it is dimagnetic otherwise paramagnetic . in this case Cl has 7 valence electrons and oxygen has 6 valence electrons . so total number of valence electrons is 7 + 6 x 2 = 19 , which is odd hence this molecule is paramagnetic . and this option is correct  .