The charges on the colloidal CdS sol and TiO$$_2$$ sol are, respectively :

The charge on a colloidal sol depends on the preferential adsorption of ions on the particle surface.

$$\text{CdS}$$ is a metal sulfide sol. Metal sulfide sols (like $$\text{As}_2\text{S}_3$$ and $$\text{CdS}$$) preferentially adsorb sulfide ions ($$\text{S}^{2-}$$) on their surface, giving them a negative charge.

$$\text{TiO}_2$$ is a metal oxide sol. Metal oxide sols (like $$\text{Fe}_2\text{O}_3$$, $$\text{Al}_2\text{O}_3$$, $$\text{TiO}_2$$) preferentially adsorb positive ions (such as $$\text{H}^+$$) on their surface, giving them a positive charge.

Therefore, $$\text{CdS}$$ sol is negatively charged and $$\text{TiO}_2$$ sol is positively charged. The correct answer is option (4): negative and positive.