Which one of the folloiwng is responsible for the stimulating effect of tea?

The stimulating effect of tea is primarily attributed to caffeine, which is a natural alkaloid compound [2]. Alkaloids are nitrogen-containing heterocyclic compounds known for their high functional activity. In tea, caffeine (sometimes referred to as theine when associated with tannins) acts as a potent stimulant of the central nervous system and the cardiovascular system. It functions mainly through adenosine receptor antagonism, which increases motor activity and arousal. While tea also contains tannins (polyphenols) and flavonoids (like catechins), these compounds are largely responsible for the tea's flavor, color, and antioxidant properties rather than its immediate stimulating effect [5]. Tannins actually bind with caffeine to slow its release, making the stimulating effect of tea softer and more prolonged compared to coffee. Therefore, the chemical class responsible for the stimulation is the alkaloid group.

Sources

1. [2] https://www.sciencedirect.com/science/article/pii/S2405844023107225
2. [1] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 12: Major Crops and Cropping Patterns in India > Table III Pharmaceutical Plants, Drugs and Use > p. 89
3. [5] https://pmc.ncbi.nlm.nih.gov/articles/PMC9557745/