In which of the following situations would exocytosis be the proper mechanism of membrane transport?

secretion of the neurotransmitter serotonin, which is a water-soluble amine molecule
secretion of a steroid hormone from a cell
secretion of water from a cell
secretion of Na+ from a cell

Secretion of the neurotransmitter serotonin, which is a water-soluble amine molecule

Explanation:

Exocytosis is defined as the process where cell transports secretary products which are packaged in transport vesicles such as antibodies, peptide hormones, secretory proteins, and enzymes with the help of cytoplasm to the plasma membrane.

Some example of exocytosis are:

1) Neurotransmitters secrets from nerve cells.

2) Antigens which helps to stimulate the immune response.

3) Proteins of the plasma membrane.

Option A, secretion of the neurotransmitter serotonin, which is a water-soluble amine molecule

Explanation:

Exocytosis is a transport mechanism used by cells to transport large molecules out of the cell though the the cytoplasm to the plasma membrane. The secreted molecules are packaged in vesicles. The neural cell’s soma releases large transmitter molecules (serotonin) until it produces stimulus modulated by active neurons, glia and blood vessels. This somatic exocytosis of serotonin  is stimulated by sequence of intracellular Ca2+ transients  that activates L-type channels thereby leading to exocytosis of serotonin from vesicles to plasma membrane

Hence, option A is correct