Which of the following is used as catalyst for the esterification of carboxylic acid and alcohol? 

The classic synthesis is the Fischer esterification, which involves treating a carboxylic acid with an alcohol in the presence of a dehydrating agent: 
RCO₂H + R′OH ⇌ RCO₂R′ + H₂O 
The equilibrium constant for such reactions is about 5 for typical esters, e.g., ethyl acetate. The reaction is slow in the absence of a catalyst. Sulphuric acid is a typical catalyst for this reaction.