What are the three primary functions of blood?

The correct choice identifies the three primary functions of blood as transportation, regulation, and protection.

Transportation refers to blood's role in transporting oxygen from the lungs to the body's tissues and carbon dioxide from the tissues back to the lungs. Additionally, blood carries nutrients from the digestive tract and hormones from the endocrine glands throughout the body. This function is essential for maintaining the homeostasis of bodily systems.

Regulation pertains to blood's ability to help regulate pH levels, body temperature, and overall fluid balance. The components of blood, such as plasma, help maintain the appropriate acidity in tissues and organs while also distributing heat generated by metabolism throughout the body.

Protection involves the immune functions of blood, primarily through white blood cells and antibodies, which help defend the body against infection, pathogens, and foreign substances. Additionally, platelets in blood play a crucial role in hemostasis, preventing blood loss through clot formation when injuries occur.

In summary, the three primary functions of blood are critical for sustaining life and maintaining the body's equilibrium, making this option the most accurate description.