Three elements X, Y and Z are in the 3rd period. The oxides of X, Y and Z, respectively, are basic, amphoteric and…

🧠 In Period 3, oxide nature shifts left-to-right: basic (metal) → amphoteric (boundary) → acidic (non-metal). So basic = lowest Z, acidic = highest Z Going left to right in Period 3: Na, Mg (basic oxides) → Al (amphoteric oxide) → Si (weakly acidic) → P, S, Cl (acidic oxides). The same direction in which oxide nature changes is the direction in which $Z$ increases.

🗺️ Place X, Y, Z X has a basic oxide → X is a metal on the left of Period 3 → low $Z$ (e.g., Na, Mg). Y has an amphoteric oxide → Y is in the middle → intermediate $Z$ (Al). Z has an acidic oxide → Z is a non-metal on the right → high $Z$ (Si–Cl range).

So: $Z(\text{X}) < Z(\text{Y}) < Z(\text{Z})$, i.e., X < Y < Z.

⚠️ The trap Don't mix up the element label Z with the atomic number notation $Z$. The question says X, Y, Z are three different elements — so "atomic number of Z" means atomic number of the third element. Read carefully so the symbols don't tangle.

$\boxed{\text{Answer: (b)}}$