If aluminium atoms replace a few silicon atoms in 3D network of silicon dioxide, the overall structure acquires a negative charge. Then, Aluminium is trivalent while silicon is tetravalent. It is so because as the equivalent number of trivalent ions like (Al⁺³) replace few tetrahedral Si atoms in the 3D structure of Silicon dioxide, the valence electron of each Si atom gets freed, thereby resulting in the formation of a negatively charged structure. Thus, SiO₂ acquires negative charge.

Therefore, both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

So, option (i) is correct.