If the pitot tube becomes completely blocked, what will happen to the airspeed reading?

When the pitot tube becomes completely blocked, the airspeed indicator will freeze at the last value it registered. This occurs because the pitot tube is responsible for measuring dynamic pressure, which is directly related to the speed of the aircraft through the air. When the tube is blocked, it can no longer measure this pressure, leading the airspeed indicator to display a constant value—the last known speed—rather than reflecting any changes in actual airspeed.

The other options describe scenarios that are not associated with a blocked pitot tube. Fluctuating readings would typically be observed with intermittent blockages or other instruments failing. There is also no mechanism in the aircraft that would automatically reduce speed in response to a faulty airspeed reading, nor would a blockage in the pitot tube directly cause errors in altitude readings, which are based on static pressure rather than dynamic pressure.