When the pitot tube becomes blocked, how does the airspeed indicator behave?

When the pitot tube becomes blocked, the behavior of the airspeed indicator is affected because it relies on the difference between static pressure and the dynamic pressure created by the airplane's speed through the air. A blockage in the pitot tube means that the airspeed indicator will not receive the dynamic pressure needed to calculate airspeed accurately.

In a climb, the static pressure decreases while the dynamic pressure remains unchanged (assuming the airspeed is constant), leading the airspeed indicator to under-read because it cannot register the drop in dynamic pressure. Conversely, during a descent, while the static pressure again decreases, the dynamic pressure will increase as the aircraft accelerates due to the downward motion, causing the airspeed indicator to over-read.

This nuanced change in readings during climbs and descents is a crucial aspect of understanding how instruments react to blockages. The other options don't accurately describe the behavior of the airspeed indicator when the pitot tube is blocked; for instance, freezing at a current speed or reading zero suggests a complete failure of the instrument, which doesn't represent the underlying physics of how static and dynamic pressures interact in varying flight conditions.