The stimulating effect of tea is mainly attributed to the presence of alkaloids. Alkaloids are a class of organic compounds that are naturally occurring and have a stimulant effect on the human body. One of the most well-known alkaloids found in tea is caffeine, which is responsible for its stimulating properties.

Option 1: Tannin - Tannins are a group of bitter and astringent compounds found in tea, but they are not responsible for its stimulating effect. Tannins are more known for their role in causing the bitter taste and astringency in tea.

Option 2: Steroid - Steroids are a class of compounds that have various physiological effects, but they are not responsible for the stimulating effect of tea. Steroids are more commonly associated with hormone regulation and muscle growth.

Option 4: Flavonoid - Flavonoids are a group of antioxidants found in tea, but they are not specifically responsible for the stimulating effect. Flavonoids contribute to the health benefits of tea, such as its antioxidant properties.

In conclusion, the correct answer is option 3: Alkaloid, specifically caffeine, is responsible for the stimulating effect of tea.