Question

which is an organ?
heart
circulatory system
blood
muscle tissue

Explanation:

An organ is a group of tissues that work together to perform a specific function. The heart is made up of different tissues (such as muscle tissue, connective tissue etc.) and its main function is to pump blood. The circulatory system is an organ - system (a group of organs working together). Blood is a type of connective tissue. Muscle tissue is just a tissue type, not an organ.

Answer:

heart