The correct answer is option 'C', the degeneration of the thymus gland is one of the reasons why old people usually exhibit weaker immune power.

The immune system is responsible for defending the body against harmful pathogens, such as bacteria, viruses, and fungi. It is composed of various organs, cells, and molecules that work together to recognize and eliminate these pathogens. The thymus gland is an essential component of the immune system, particularly in the early stages of life.

The thymus gland is located in the upper chest region, just behind the breastbone. It plays a crucial role in the development and maturation of T lymphocytes, also known as T cells. T cells are a type of white blood cell that is responsible for cell-mediated immunity, which involves direct cell-to-cell interaction to eliminate pathogens.

During childhood, the thymus gland is highly active and reaches its maximum size. However, as a person ages, the thymus gland gradually decreases in size and becomes replaced by fat tissue. This process is known as thymic involution and is a natural part of aging.

The decrease in thymus gland activity and size results in a decline in the production and maturation of T cells. T cells are crucial in recognizing and eliminating pathogens, so a reduction in their numbers and function can weaken the immune response.

In addition to thymic involution, aging also affects other components of the immune system, such as a decrease in the production of antibodies by B cells and a decline in the function of natural killer cells. These age-related changes collectively contribute to a weaker immune system in older individuals.

It is important to note that while aging does lead to changes in the immune system, not all elderly individuals will have a weaker immune response. Factors such as overall health, lifestyle, and genetics can also influence immune function in older adults.

In conclusion, the degeneration of the thymus gland is one of the reasons why old people usually exhibit weaker immune power. The decrease in thymus gland activity and size leads to a decline in the production and maturation of T cells, which are crucial for an effective immune response. However, it is important to consider that aging affects the immune system in various ways, and individual differences can also play a role in immune function in older adults.