Dear Student,
The answer to the question is "Option C".
The "Thymus Gland" is the largest gland in an human body till the puberty, after that it starts shrinking and by the age of 75, it is nothing but a fatty issue. The function of the thymus gland is secretion and maturation T-lymphocyte cells for our defence against viruses & bacteria, and by puberty it has made all the T cells.
Hence, it decreases with age.
Regards