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An organelle can be defined as a subcellular structure that performs one or more specific functions in the cell, just like an organ does in the body. Mitochondria, nuclei are the important organelles present in the cell. They perform various important functions like producing energy and storing genetic information respectively.

Gravitropism is the plant's response to the gravitational force of the earth. It ensures the shoot grows upwards (negative gravitropism) and the root grows downward (positive gravitropism). Amyloplasts are otherwise known as statoliths are mainly specialized plastids containing starch granules and in response to gravity, it settles downward. Amyloplasts are present in specialized cells of the root cap and also present in shoots. Their presence helps the plant to grow vertically. A mutant root of the plant lacks amyloplasts and therefore causes the root to grow in all directions.

Mitochondria are energy-producing organelles and do not play a role in root growth.

Chloroplast is involved in the food synthesis process in plants.

The nucleus helps in storing genetic materials.

Therefore option ‘b’ is the correct answer.

In absence of amyloplast, roots grow in all directions that are found in mutant plants.