Questions: Cells of the body mainly use for the selective endocytosis of most fluid-phase endocytosis receptor-mediated endocytosis diffusion pinocytosis

The answer is: receptor-mediated endocytosis.

Explanation for each option:

1. Fluid-phase endocytosis: This is a non-selective process where cells engulf extracellular fluid and its dissolved solutes. It is not the primary method for selective uptake of specific molecules.

2. Receptor-mediated endocytosis: This is a highly selective process where cells use receptor proteins on their surface to bind specific molecules (ligands) and then internalize them. This is the correct answer as it is the main method cells use for selective endocytosis.

3. Diffusion: This is a passive process where molecules move from an area of higher concentration to an area of lower concentration. It is not a form of endocytosis and does not involve the selective uptake of molecules.

4. Pinocytosis: This is a form of endocytosis where cells engulf extracellular fluid and its dissolved solutes. It is similar to fluid-phase endocytosis and is not selective.

5. IDON'T KNOWYET: This is not a valid scientific term or process related to endocytosis.

Summary: Cells of the body mainly use receptor-mediated endocytosis for the selective endocytosis of most molecules. This process involves specific receptors on the cell surface that bind to target molecules, allowing for their selective internalization.