when acetaldol is treated with excess of acid then unsaturated product will be  
a) alcohol
b) aldehyde
c) acid
d) alkyl halide 

Dear student,

Acetaldol on reaction with an acid will yield Crotonaldehyde which is an aldehyde.

This is a simple dehydration reaction and H₂SO₄ is a good dehydrating agent.

Hope this helps,
 

Regards