In multicellular organisms, the mechanism most directly responsible for directing development and maintaining homeostasis is gene:
a. expression.
b. duplication.
c. deletion.
d. regulation.

In multicellular organisms, the mechanism most directly responsible for directing development and maintaining homeostasis is gene "regulation"

Explanation:

Even though the organs present throughout the body helps in maintaining the homeostasis, But the systems like endocrine system and nervous system plays important role in sustaining and regulating it.  The gene regulation is the increase and decrease of the specific gene products. Gene regulation can also be understood as the regulating process which helps in controlling ability of the cell to the environmental changes or can say the adaptability of the cell to changing environment done by gene regulation.

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.