Which substance is primarily responsible for carrying oxygen in the blood?

Hemoglobin is the primary substance responsible for carrying oxygen in the blood. This protein is found in red blood cells and has a unique ability to bind oxygen molecules in the lungs and release them in tissues where oxygen is needed. Hemoglobin contains iron, which gives it the capacity to bind with oxygen efficiently, facilitating the transport of this essential gas throughout the body.

Glucose, on the other hand, is a simple sugar that serves as a primary energy source for cells but does not have a role in oxygen transport. Insulin is a hormone that regulates blood sugar levels and does not participate in the transport of oxygen. Serum albumin is a protein that helps maintain the osmotic pressure of blood and carries various substances, but it is not involved in oxygen transport like hemoglobin is. Thus, hemoglobin's specialized function in oxygen transport makes it the correct answer to the question.