People that are confined to bed for long periods of time often develop edema because
a. their blood pressure becomes elevated, forcing more fluid into interstitial spaces as blood hydrostatic pressure rises.
b. lack of motor activity leads to reduced sympathetic stimulation to lymphatic vessels, so lymph tends to pool.
c. without skeletal muscle contraction to force lymph through lymphatic vessels, fluid tends to accumulate in the interstitial spaces.
d. reduced vasomotor tone allows proteins to leak from plasma, and water follows the osmotic gradient.

A. their blood pressure becomes elevated, forcing more fluid into interstitial spaces as blood hydrostatic pressure rises.

Explanation:

Edema is characterized by an increase in the amount of interstitial fluid in a tissue or within a cavity. The fluid accumulated in edema is formed by an aqueous solution of salts and proteins from the blood plasma.  An edema can have various causes, such as changes in the wall of a capillary, decreased plasma oncotic pressure, increased hydrostatic blood pressure, sodium retention, and decreased lymphatic drainage.

In people who are confined to bed for long periods of time they usually develop edema because blood pressure rises, forcing more fluid into the interstitial spaces as hydrostatic blood pressure increases.