Borax bead test is performed only for coloured salt. Borax (sodium pyroborate), Na₂B₄O₇.10H₂O on heating gets fused and lose water of crystallisation. It swells up into fluffy white porous mass which melts into a colourless liquid which later form a clear transparent glassy bead consisting of boric anhydride and sodium metaborate.



Boric anhydride is non-volatile. When it react with Cr(III) salt then deep green complex is formed.



Hence, option (c) is correct.