What could a pilot observe that indicates the presence of carburetor icing?

A decrease in engine performance can indicate the presence of carburetor icing because carburetor ice forms when moisture in the air freezes in the carburetor, disrupting the airflow and fuel mixture. This obstruction affects engine performance, causing a noticeable loss of power and responsiveness. As the ice builds up, it hinders the engine's ability to operate efficiently, leading to reduced RPMs and diminished thrust. Pilots often monitor engine performance as a vital indicator of potential issues, and a drop in performance commonly serves as a primary warning sign of carburetor icing.

The other options do not directly relate to carburetor icing in the same way. Increased fuel consumption could occur due to other reasons such as engine mismanagement; smoke from the exhaust generally signals combustion inefficiencies or mechanical problems rather than ice formation; and a rapid climb rate would typically indicate good engine performance, which would be inconsistent with the presence of carburetor icing.