Which of the following was not present in the primitive atmosphere of…

The correct answer is Option (4) → Free oxygen

There was no atmosphere on the early earth. Water vapour, methane, carbon dioxide and ammonia were released from the molten mass and covered the surface. The oxygen set free when UV rays split water combined with ammonia and methane, so the atmosphere stayed reducing, with no free oxygen.

Option (1) Water — Incorrect. Water vapour was released from the molten mass, and it later fell as rain to form the oceans.

Option (2) Methane — Incorrect. Methane (CH₄) was present; it is one of the gases of the reducing atmosphere.

Option (3) Ammonia — Incorrect. Ammonia (NH₃) was present too, alongside methane.

Option (4) Free oxygen — Correct. Free oxygen was absent. The oxygen formed from water combined with ammonia and methane, and the atmosphere was reducing.

NCERT Extract:
Origin of Life – “… There was no atmosphere on early earth. Water vapour, methane, carbondioxide and ammonia released from molten mass covered the surface. The UV rays from the sun brokeup water into Hydrogen and Oxygen and the lighter H₂ escaped. Oxygen combined with ammonia and methane to form water, CO₂ and others.”
Origin of Life – “… The conditions on earth were – high temperature, volcanic storms, reducing atmosphere containing CH₄, NH₃, etc.”