Consider the oxides $\mathrm{Al}{}_2\mathrm{O}{}_3$, $\mathrm{SiO}{}_2$, $\mathrm{P}{}_4\mathrm{O}{}_{10}$ and $\mathrm{Cl}{}_2\mathrm{O}{}_7$. The correct increasing order of acidic character is

Across period 3 the electronegativity and the oxidation state of the element both increase, so the acid character of the oxides rises. $\mathrm{Al}{}_2\mathrm{O}{}_3$ is amphoteric, $\mathrm{SiO}{}_2$ weakly acidic, $\mathrm{P}{}_4\mathrm{O}{}_{10}$ strongly acidic and $\mathrm{Cl}{}_2\mathrm{O}{}_7$ the most acidic, giving the order in the first option.