Anomalous behaviour of oxygen is due to its

Oxygen shows anomalous behaviour compared to other members of Group 16 (S, Se, Te, Po). This is primarily due to two factors:

1. Small size:

Oxygen is the first element in Group 16 and has a very small atomic radius. Due to its small size:

- It has a high ionization enthalpy

- It has strong interelectronic repulsion in its compact orbitals

- It cannot expand its octet (no d-orbitals available in the second shell)

- Its maximum covalency is limited to 4 (unlike S which can show covalency of 6)

2. High electronegativity:

Oxygen is the second most electronegative element (after fluorine). Due to its high electronegativity:

- It forms strong hydrogen bonds

- It primarily shows a $$-2$$ oxidation state (unlike S, Se which show $$+2, +4, +6$$ as well)

- It tends to form $$p\pi - p\pi$$ multiple bonds with itself and with other small atoms like C and N

These two properties together make oxygen's behaviour significantly different from the rest of its group members.

The correct answer is Option 3: Small size and high electronegativity.