Hi there!
The answer is C. nicotine.
Tobacco is the carrier of nicotine. This stimulates the brain but also makes you dependent, which can eventually cause (life-long) addiction.
Question's pretty vague, although I recommend a name that's brief and easily recognizable as well as something that relates to the purpose. So if someone sees it, they can easily remember and identify what it is exactly.
Correct answers
D
Ventricles
The left ventricles pump the blood out of the heart to the whole body.
They dont because its not a state licensed profession