Amorphous boron is extracted from borax by the following steps :
`"Borax" overset((A))rarr H_(3) BO_(3) overset("Heat")rarr B_(2) O_(3) overset((B)) rarr "Boron" (A) and (B) are`
(a) `H_(2) SO_(4), Al`
(b) `HCl`, Carbon
( c) `H_(2) SO_(4), Mg`
(d) `HCl, Fe`.

Explanation

Boric acid, `H_(3) BO_(3)`, is formed by acidification of hot concentrated solution of borax by hot and conc solution of `H_(2) SO_(4)`.
`Na_(2)B_(4)O_(7) +5H_(2) SO_(4) + 5H_(2) O rarrNa_(2) SO_(4) + 4H_(3) BO_(3)`
`{(H_(3) BO_(3) overset(Delta)rarr B_(2) O_(3) + H_(2) O,,,,),(B_(2) O_(3) + 3Mg rarr 2B + 3MgO,,,,):}}`
`B_(2) O_(3)` is reduced to `B` by strong electropositive element only. Hence `(A)` is `H_(2) SO_(4)` and `(B)` is `Mg`.
Therefore, option `(c)` is correct.