The answer is indeed (4) although this arguably isn’t a very well-set question since it does require some knowledge that is outside the primary school syllabus.

In general:

Circulatory system – transports nutrients to all parts of the body through a network of blood vessels

Digestive system – helps to break down food into simpler substances so it can be absorbed into the body

Respiratory system – regulates the flow of oxygen which is then carried to all parts of the body by the circulatory system

Skeletal system – marrow inside our bones produces the red blood cells. The red blood cells are responsible for giving oxygen to the tissues in exchange for carbon dioxide

Muscular system – general movement helps the circulatory system to function effectively. A more specific instance of them working together is when the muscles in the legs expand and contract accordingly to help to pump de-oxygenated blood back to the heart (to counter gravity)