Solve 19th one and give proper explanation
Q.19. The number of OH units directly linked to boron atoms in $N{a}_2{B}_4{O}_7.10H_{2}O$ is
(a) 2
(b) 3
(c) 4
(d) 10

Dear student,
Please refer to the structure of Borax or Na₂B₄O₇.10H₂O .
It can also be given as Na₂[B₄O₅(OH)₄].8H₂O



Hence it has 4 OH directly linked with boron.
C is the answer.

Hope it helps
Regards