When a cytoplasmic vesicle fuses with the plasma membrane, expelling its contents outside the cell, the process is known as:
a. active transport
b. exocytosis
c. phagocytosis
d. endocytosis?

The correct answer is b. Exocytosis.

Exocytosis is the form of active transport and fusion of secretory vesicles with the plasma membrane and the extracellular space. With this process cell transports molecules like neurotransmitters and proteins out of the cell. This exocytosis is possible because the vesicle transiently fuses with the outer cell membrane.Opposite process of the exocytosis is endocytosis (transport of the molecules into the cell).

that is wrong, its not thymus.