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The primary source of nitrogen for the proteins in our body tissues is nitrogen:

A released as a gas by volcanoes.

B found in plant and animal proteins we eat.

C released by dead and decaying organisms into the air.

D absorbed directly from the air.

The primary source of nitrogen for the proteins in our body tissues is nitrogen  found in plant and animal proteins we eat.

Option B

Explanation:

To cope for living, plants also in the requirement of nitrogen. So, they used to absorb nitrogen from soil compounds such as nitrates, ammonia, and nitrites. Some plants - usually legumes, some trees and shrubs such as birch and alder - consists symbiotic bonding with bacteria; Microorganisms bind to the plants root and form nitrogen in the soil from nitrogen gas.

Plants use nitrogen to make proteins, amino acids, enzymes, and nucleotides (DNA components). These consumed by all people when they eating plants or animal meat. People cannot consume nitrogen by breathing, but can consume it by eating plants or animals that have consumed nitrogen-rich vegetation.

The air we breathe is 78% nitrogen. Of course, it gets into our body with each breath. Because nitrogen is an important part of human health, it disappoints that those who inhale nitrogen and immediately exhale. Animals, including humans, cannot absorb nitrogen as gas.