Step 1: Understanding the Concept:
Dihydric phenols are benzene derivatives with exactly two hydroxyl ($-\text{OH}$) groups.
Step 2: Key Formula or Approach:
Check the number of $-\text{OH}$ groups in the structures:
Dihydric $=$ 2 groups, Trihydric $=$ 3 groups.
Step 3: Detailed Explanation:
- Catechol: Benzene-1,2-diol (2 groups).
- Resorcinol: Benzene-1,3-diol (2 groups).
- Hydroquinone: Benzene-1,4-diol (2 groups).
- Phloroglucinol: Benzene-1,3,5-triol. It has 3 hydroxyl groups, so it is a trihydric phenol, not dihydric.
Step 4: Final Answer:
Phloroglucinol is NOT a dihydric phenol.