According to one of the most widely accepted theories, the Earth's atmosphere before the origin of life was reducing with oxygen absent in O2 form. Let's explore this answer in detail:

Theories about the Earth's Early Atmosphere:

There are several theories proposed to explain the composition of the Earth's early atmosphere. These theories are based on geological evidence, chemical analyses of ancient rocks, and extrapolation from the atmospheres of other planets in our solar system.

1. Oxidizing Atmosphere:

Some scientists initially believed that the Earth's early atmosphere was oxidizing, meaning it contained a significant amount of oxygen. However, this theory has been largely discounted because the presence of oxygen would have prevented the formation of certain organic molecules necessary for the origin of life.

2. Oxidizing Atmosphere with H2:

Another theory suggests that the Earth's early atmosphere was oxidizing but also contained hydrogen gas (H2). This hypothesis is based on the idea that volcanic activity released large amounts of water vapor, which was dissociated into hydrogen and oxygen by intense ultraviolet radiation. However, this theory also faces challenges in explaining the origin of life.

3. Reducing Atmosphere with Free O2 in Small Amount:

Some researchers propose that the Earth's early atmosphere was reducing, meaning it lacked oxygen and contained gases such as methane (CH4), ammonia (NH3), water vapor (H2O), and hydrogen gas (H2). This reducing atmosphere is thought to have provided the necessary conditions for the synthesis of organic molecules, including the building blocks of life. However, the presence of free oxygen in small amounts makes this theory less plausible.

4. Reducing Atmosphere with Oxygen Absent in O2 Form:

The most widely accepted theory suggests that the Earth's early atmosphere was reducing, with oxygen absent in the form of O2. Instead, the atmosphere likely contained gases such as methane, ammonia, water vapor, and hydrogen gas. The absence of O2 allowed for the synthesis of organic molecules through various chemical reactions, including the Miller-Urey experiment, which demonstrated that these gases could react to form amino acids, the building blocks of proteins.

Conclusion:

In conclusion, the correct answer is option 'D' - the Earth's early atmosphere was reducing with oxygen absent in the form of O2. This theory is widely accepted because it provides the most favorable conditions for the origin of life on Earth. The absence of free oxygen allowed for the synthesis of organic molecules, setting the stage for the development of life as we know it.