What is the primary effect of a blocked static port on the airspeed indicator?

The primary effect of a blocked static port on the airspeed indicator is that it gives erroneous speed readings. The airspeed indicator measures dynamic pressure, which is the difference between the total pressure (from the pitot tube) and static pressure (from the static port). When the static port is blocked, the airspeed indicator cannot receive accurate static pressure information, which is used to calculate the aircraft's airspeed.

As altitude changes, the static pressure outdoors also changes. If the static port is blocked, the airspeed indicator will not be able to adjust its readings according to the changes in atmospheric pressure. Instead, it will respond to changes in total pressure, leading to incorrect airspeed indications that may either increase or decrease with altitude. This erroneous behavior can be critical in flight, as pilots rely on accurate airspeed information for safe operation.

The other options relate to different outcomes that do not accurately describe the scenario of a blocked static port. While the airspeed indicator does not become completely inoperative or display a constant speed, it will certainly not show the same altitude continuously — these behaviors would not appropriately represent the effects of a blocked static port. Hence, the only accurate outcome of the blockage is that it results in erroneous speed readings.