Oxygen was first released in the atmosphere by the activity of:

The correct answer is Option (1) → Earliest cells capable of performing reactions similar to light reactions of photosynthesis

The first cells appeared on earth about 2000 million years ago. Some of them could split water with the help of solar energy, which they caught with light-harvesting pigments. This is the light reaction of photosynthesis, and it releases oxygen.

(1) Earliest cells capable of performing reactions similar to light reactions of photosynthesis — Correct. Their light-reaction-like activity put the first oxygen into the atmosphere.

(2) Photosynthesis by decomposers — Incorrect. Decomposers, such as bacteria and fungi, feed on dead matter. They do not make food by photosynthesis.

(3) Photosynthesis by chemoautotrophs — Incorrect. Chemoautotrophs get energy from chemical reactions, not from light, so they do not carry out photosynthesis.

(4) Oxygen is released during the light-independent reactions of photosynthesis — Incorrect. Oxygen comes from the splitting of water in the light reactions. The light-independent reactions only fix carbon dioxide into sugar.

NCERT Extract:
A Brief Account of Evolution – “… About 2000 million years ago (mya) the first cellular forms of life appeared on earth. The mechanism of how non-cellular aggregates of giant macromolecules could evolve into cells with membranous envelop is not known. Some of these cells had the ability to release O2. The reaction could have been similar to the light reaction in photosynthesis where water is split with the help of solar energy captured and channelised by appropriate light harvesting pigments.”