If a pilot suspects that the engine (with a fixed-pitch propeller) is detonating during climb-out after takeoff, the initial corrective action to take would be to

Explanation

Detonation during climb-out is driven by high cylinder temperatures; PHAK Chapter 7 lists lowering the nose to increase airspeed and cooling airflow, along with enriching the mixture and reducing power, as ways to stop it. Leaning the mixture raises temperatures and makes detonation worse, and carburetor heat raises the intake air temperature.