The major building block of an organism is…

Explanation

Carbon is the primary building block of life on Earth.Carbon can form stable, complex molecules with other elements, especially hydrogen, oxygen, and nitrogen.

Carbon-based molecules are the basis for life in all living organisms, including humans. These molecules include nucleic acids, carbohydrates, lipids, cells, and proteins. It is considered the major building block of organisms because of its unique ability to form stable bonds with many different elements, including itself, allowing it to create complex, diverse, and large molecules necessary for life, like proteins, carbohydrates, and nucleic acids, which are the foundation of all living things. 

There is an explanation video available .