Define cell.
A. A thick solution mainly composed of water, salts, and proteins.
B. Basic structural, functional, and biological unit that can replicate independently.
C. Biopolymers or large biomolecules essential for all known forms of life.
D. All of the above.

Hint: Cells are the smallest unit of life that replicate independently and are called the building blocks of life. The human body is made up of trillions of cells.

Complete step by step answer: A cell is the structural and fundamental unit of life. The study of cells from its basic structure to the functions of every cell organelle is called Cell Biology. Robert Hooke was the first Biologist who discovered cells. All organisms are made up of cells. They may be made up of a single cell (unicellular), or many cells (multicellular). Mycoplasmas are the smallest known cells. Cells are the building blocks of all living beings. They provide structure to the body and convert the nutrients taken from the food into energy. Cells are complex and their components perform various functions in an organism. They are of different shapes and sizes, pretty much like bricks of the buildings. Our body is made up of cells of different shapes and sizes. Cells are the lowest level of organization in every life form. From organism to organism, the count of cells may vary. Humans have a number of cells compared to that of bacteria. Cells comprise several cell organelles that perform specialized functions to carry out life processes. Every organelle has a specific structure. The hereditary material of the organisms is also present in the cells.
Hence, the correct answer is Option B

Note: Cell contains a fluid called the cytoplasm, which is enclosed by a membrane. Also present in the cytoplasm are several biomolecules like proteins, nucleic acids, and lipids. Cellular structures are called cell organelles that are suspended in the cytoplasm.