The red solid is aluminium (remember that amphoteric oxides can react with bases).

Ar of aluminium is 27.
Ar of oxygen is 16.
Mr of aluminium oxide is 102.

In 5.4 g aluminium, equivalent amount in moles
= 5.4 / 27
= 0.2 mol

Every 1 molecule Al2O3 results in 2 atoms of Al coming out, so by this molar ratio 1 : 2, we realise that 0.2 mol Al comes from 0.1 mol Al2O3.

Equivalent mass of Al2O3
= 0.1 x 102
= 10.2 g

But the mass of the sample is 12 g.

Means, only 10.2 g out of the 12 g is made of aluminium oxide.

Percentage purity
= 10.2 / 12 x 100%
= 85%

So, none of the four options are correct.