The correct answer is D. Caffeine.
Caffeine is a naturally occurring alkaloid found in coffee beans, tea leaves, cocoa beans, and kola nuts. It is a stimulant that increases alertness and wakefulness. Caffeine is also used in many over-the-counter medications to treat headaches and pain.
Cocaine is a stimulant drug that is extracted from the leaves of the coca plant. It is a powerful central nervous system stimulant that can cause addiction. Cocaine is illegal in most countries.
Morphine is an opioid drug that is used to relieve pain. It is derived from the opium poppy. Morphine is a Schedule II drug in the United States, which means that it has a high potential for abuse and addiction.
Tannin is a type of polyphenol that is found in many plants. It is a bitter, astringent substance that is used in tanning leather and in some beverages. Tannin is not a stimulant.