When borax is heated with CoO on a platinum loop, blue coloured bead formed is

1. $Co\left[B_4{O}_5\right(OH{)}_4]$
2. $Co(B{O}_2{)}_2$
3. $B_2{O}_3$
4. $Co{B}_4{O}_7$

$N{a}_2{B}_4{O}_7\cdot 10H_{2}O\stackrel{\Delta }{\to }N{a}_2{B}_4{O}_7+10H_{2}O$

$N{a}_2{B}_4{O}_7\stackrel{\Delta }{\to }NaB{O}_2\left(sodiummetaborate\right)+B_2{O}_3$

$B_2{O}_3+CoO⟶\underset{BlueBead}{Co(B{O}_2{)}_2}\left(cobalt\right(II\left)metaborate\right)$