Where is the thymus located?
a. posterior to the stomach
b. superior to the heart
c. near lymphatic capillaries
d. posterior to the sternum

d. Posterior to the sternum.

Explanation:

The thymus is a very large gland active during childhood, that is located right behind the sternum, in the middle of the two lungs. This area is part of the Mediastinum. The Mediastinum is a region inside the thorax that contains the great vessels (aorta artery, pulmonary artery), the heart, the trachea, the esophagus, the thymus, and many lymph nodes.

The thymus is important because before birth and during childhood produces a substance called thymosin that stimulates the production of lymphocytes.

The thymus cannot be posterior to the stomach, because in this location is the peritoneum that is a thick layer of fatty tissue with the function of sticking the abdominal organs to its place.

The thymus cannot be superior to the heart, because right there we could find the aorta artery and pulmonary veins which are big enough to fill that place.

Near lymphatic capillaries is not a place for a big gland as the thymus. Because these tiny structures are located in the intercellular space, so it is too small to have the thymus in it.