Permitting the exchange of nutrients and gases between blood and tissue cells is the primary function of question 4 options:

This describes the function of the capillaries. Capillaries are the smallest of the body's blood vessels. They are one cell thick, and are the sites of the transfer of oxygen and other nutrients from the bloodstream to other tissues in the body. Additionally they collect carbon dioxide, waste materials and fluids for return to the veins. They connect the tiny muscular branches of arteries called arterioles, with tiny veins called venules. 

The answer is option a.