If an aircraft is equipped with a fixed-pitch propeller and a float-type carburetor, the first indication of carburetor ice would most likely be

Explanation

PHAK Ch. 7 states that in an airplane with a fixed-pitch propeller the first indication of carburetor ice is a loss of RPM, because ice restricts the fuel/air flow and reduces power; engine roughness typically follows later. A drop in oil and cylinder head temperature is not the recognized first sign. With a constant-speed propeller the first sign would instead be a drop in manifold pressure.