$SnO_2$ forms a positively charged colloidal Sol in the basic medium why ?

Question

$\begin{array}{1 1} \text{(a) $SnO_2$ is amphoteric in nature. It reacts with acid eg HCl to form $SnCl_4$ in the solution . The common $Sn^{4+}$ ions are adsorbed on the surface of $SnO_2$ particles giving the, a positive charge}\\ \text{(b) $SnO_2$ reacts with a base eg. $NaOH$ to form sodium stannate in the solution.The stannate ions are adsorbed on the surface of $SnO_2$ particles giving them a Positive charge .} \\ \text{(c) $SnO_2$ reacts with a base eg. $HCl$ to form sodium stannate in the solution.The stannate ions are adsorbed on the surface of $SnO_4$ particles giving them a positive charge .}\\\text{(d) None of the above} \end{array}$