Which one amongst the following has highest ionization enthalpy?

The correct answer is Option (1) → oxygen

Ionisation enthalpy decreases down a group: the atomic size increases and the outer electron is farther from the nucleus and better shielded, so it is removed more easily. Group 16 values (kJ mol⁻¹):

    O 1314 > S 1000 > Se 941 > Te 869

So oxygen, the smallest atom of the group, has the highest ionisation enthalpy.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Physical Properties – …