What is a vesicle?

A vesicle is correctly defined as a small sac containing fluid. In biological terms, vesicles are membrane-bound structures that transport materials within cells or from one cell to another. They can carry a variety of substances, including proteins, nutrients, and waste products, and play a crucial role in processes such as metabolism, communication, and storage within the cell. This definition aligns perfectly with the accepted scientific understanding of vesicles, making option A the accurate choice for this question.

While internal organs, adhesive materials, and cellular structures are important in their contexts, they do not describe the specific nature of a vesicle. A vesicle's primary function is related to its role as a fluid-filled sac, differentiating it clearly from those other categories.