Answer;
A. Blood
Explanation;
The main organs of the Circulatory system are the heart, the brain, the lungs. There are other key things like arteries, veins, and capillaries but they are not organs.
The major tissues in the circulatory system are the epithelial, muscle, nervous, and connective tissues.
Blood is the main connective tissue in the circulatory system, but it's a liquid connective tissue. Blood tissue is in the same family as bone and cartilage, but instead it has a liquid medium that holds it together.