x, y and z are three elements of 3rd period such that their oxides are acidic, amphoteric and basic respectively. The order of atomic number of the elements is:

The correct option is A x > y > z
On moving left to right in a period, metallic nature of elements decreases and so acidic character increases. The element in the left side in the 3rd period is more metallic(form basic oxide) and in the right side, it is more non-metallic(form acidic oxide).
Atomic number increases from left to right in a period.
The order of oxides for 3rd period elements:

$\underset{Basic}{\underset{}{N{a}_{2}OMgO}}\underset{Amphoteric}{\underset{}{A{1}_2{O}_3}}\underset{Acidic}{\underset{}{Si{O}_2{P}_2{O}_{5}C{1}_2{O}_7}}$

From left to right,
(i) Acidic character $\uparrow$
(ii) Atomic number$\uparrow$
So z has minimum atomic number & x has maximum atomic
number.
So correct order of atomic number is x > y >z
Hence, (a) is the correct option.