The endocrine system acts through chemical messengers called- -
a. tissue
b. blood cells
c. neurotransmitters
d. hormones

The best answer is D.

Hormones are chemical messengers  secreted mostly into blood and also extracellular fluid by specialized tissue of the endocrine system and affect the functioning of other tissues or cells.

Most hormones circulate in blood and come in contact with essentially all cells, but a particular hormone will only affect a limited number of cells, which are termed as target cells to that hormone.

A target cell responds to a particular  hormone because it bears receptors for that hormone.

For example the red blood cells have receptors for the  hormone called insulin which is produced by the pancreas. The red cells are able to take in glucose  when their receptors bind to insulin. 

the calvin cycle has three stages. in stage 1, the enzyme rubisco incorporates carbon dioxide into an organic molecule. in stage 2, the organic molecule is reduced. in stage 3, rubp, the molecule that starts the cycle, is regenerated so that the cycle can continue