Assertion (A) : Phenol undergoes Kolbe’s reaction  wheras ethanol  does not

Reason (R) : Phenoxide ion is more basic than ethoxide ion.

ethoxide

Reason is wrong.Assertion is true.

In Phenoxide ion the lone pair on Oxygen are in conjugation with Benzene ring whereas in Ethoxide ion the electron density at Oxygen is increased due to Inductive effect of ethyl group making it more basic(Ofcourse basicity also depends on many other factors!).

Just refered mechanism .