Hemoglobin carries oxygen to the body's cells. which system contains hemoglobin?
a. nervous
b. endocrine
c. respiratory
d. circulatory

The circulatory system, as it circulate the hemoglobin which contains the oxygen you need to survive, everywhere in you body.

It wouldn't be nervous, because this is what regulates you thoughts, the impulsions of your actions etc. but it's main function is more like giving orders than executing.

Respiratory gets the oxygen but cannot give it, for example, directly to your kidneys, the circulatory system is the intermediate between those two.

The circulatory system contains hemoglobin. 

The red blood cells that transport oxygen and travel from our lungs to our cells, incorporates a protein that is named hemoglobin. Hemoglobin carries the oxygen throughout our body. The topmost percent of oxygen is carried through hemoglobin while the other percent is dissolved.

You need to put a picture of what you want answered

The correct answer would be: c. ritalin