What is it called when a structure increases in size due to the growth of individual cells?

The correct choice pertains to hypertrophy, which refers to the increase in the size of a structure or an organ due to the enlargement of its individual cells. This physiological process can occur in various tissues, such as muscle tissue, where the muscle fibers grow larger in response to increased demands, such as through strength training.

Hypertrophy can be seen in scenarios where there is an increased workload or exercise, leading to more substantial and thicker muscle fibers. This process does not involve an increase in the number of cells, which distinctly differentiates it from hyperplasia—where the total number of cells increases, resulting in the tissue's growth.

In contrast, hypoplasia refers to an underdevelopment or incomplete development of a tissue or organ, leading to a smaller size than normal. Aplasia indicates a complete failure of development or absence of an organ or tissue. Thus, hypertrophy is specifically concerned with the growth of existing cells, making it the correct answer in the context of the question presented.