What is the main function of the circulatory system in multicellular organisms?
a.
to bring oxygen into the lung
b.
to put carbon dioxide into all blood vessels
c.
to expel waste products from the body
d.
to transport materials throughout the body

The answer here is D because it covers the other options namely A and C while B is not really true.
Multi cellular organisms are made up of billions of cells each specialized to a unique role and these cells are organized into structural and functional units such as tissues and organs.
For each cell and organ to function optimally, it requires oxygen and nutrients and needs carbon dioxide and  metabolic waste taken away  in a  continuous, never ending  process.
A mechanism  is needed to accomplish the important function of transporting the relevant materials to and from the cell. The circulatory system fits this  bill splendidly. It is a specialized organ system which allows blood  and lymph to flow through the entire body carrying nutrients such as glucose, amino acids and electrolytes as well as oxygen to each cell, at the same time carting away carbon dioxide and all sorts of metabolic waste over to excretory organs  to be expelled from the body.