The correct answer is Option (4) –Free oxygen
Free oxygen (O2) was not present in the primitive Earth's atmosphere.
The primitive Earth's atmosphere was predominantly composed of gases like water vapor (H2O), methane (CH4), ammonia (NH3), carbon dioxide (CO2),and traces of hydrogen (H2). However, free oxygen was absent in the early stages of the Earth's atmosphere. Molecular oxygen began to accumulate in significant amounts later, primarily due to the development of oxygenic photosynthesis by early photosynthetic organisms.
Therefore, among the options provided, free oxygen was not present in the primitive Earth's atmosphere.
