Step 1: Understanding the Question:
A trihydric phenol is a compound where three hydroxyl (\( -OH \)) groups are directly attached to a benzene ring.
Step 2: Detailed Explanation:
1. Catechol: Benzene-1,2-diol (Dihydric).
2. Resorcinol: Benzene-1,3-diol (Dihydric).
3. Quinol (Hydroquinone): Benzene-1,4-diol (Dihydric).
4. Pyrogallol: Benzene-1,2,3-triol (Trihydric). It has three \( -OH \) groups.
Step 4: Final Answer:
Pyrogallol is the trihydric phenol among the options.