Three elements $X. Y$ and Z are in the $3^{rd}$ period of the periodic table.

The oxides of $X. Y$ and $Z$. respectively. are basic, amphoteric and acidic.

 The correct order of the atomic numbers of $X. Y$ and $Z$ is

On moving left to right in a period. Acidic character of oxides is increase.

$3^{rd}$ period element oxides

$\underset{Basic}{\underbrace{N{a}_{2}O MgO }}\underset{Amphoteric }{\underbrace{A{l}_2{O}_3}}\underset{Acidic}{\underbrace{Si{O}_2 P_2{O}_5 C{l}_2{O}_7}}$

(i) Acidic Character $\uparrow$

(i) Atomic no $\uparrow$

So $X$have minimum Atomic No 

and $Z$ have maxima Atomic No

So correct order is $X<Y<Z$