Concept: 

• This is a type of Reduction Reaction. 
• The reduction is a chemical reaction that involves the gaining of electrons by one of the atoms involved in the reaction between two chemicals.
• Oxidation is the gain of oxygen and reduction in loss of oxygen.​

Explanation:

• When phenol is reacted with zinc dust it forms benzene with zinc oxide as a side product.
• It is because zinc dust is a strong reducing agent. 
• Zinc oxidized itself and become ZnO and reduces phenol to benzene.
• NaOH is just a base and H2SO4 is a strong oxidizing agent and acid.

​Additional Information

Phenol to C6H5COOH (Benzoic acid):

Phenol to C6H5CHO (Benzaldehyde)