Final answer:
Carburetor icing in a constant speed propeller system is indicated by a decrease in engine manifold pressure (MP) while the RPM remains controlled by the system.
Step-by-step explanation:
Carburetor icing in an aircraft with a constant speed propeller can be detected when observing manifold pressure indications. When ice begins to build up in the carburetor, it restricts airflow to the engine. The reduced airflow leads to a drop in engine manifold pressure (MP), while the RPM is maintained by the constant speed propeller system. Therefore, the correct answer to how carb ice might be detected when observing manifold pressure indications is: b) MP would decrease.