If the vent pipe of an aircraft's fuel tank becomes blocked, it will cause:

When the vent pipe of an aircraft's fuel tank becomes blocked, it prevents the normal flow of air into the tank while fuel is being consumed. As fuel is drawn from the tank during operation, the volume of liquid decreases, which would typically create a vacuum or negative pressure inside the fuel tank. This loss of air pressure in the tank can lead to an undesirable situation where the pressure inside the tank drops, potentially causing fuel flow issues.

This understanding is critical as it highlights how the venting system plays an essential role in maintaining proper atmospheric pressure balance in the fuel tank. Without adequate venting, the tank cannot equalize, leading to the aforementioned pressure drop as fuel is used. Conversely, if the tank were not vented properly, one might expect an increase in pressure, but that isn't the case with a blocked vent.

Other options suggest different dynamics of pressure or fuel flow that do not correctly reflect the impact of a blocked vent. Specifically, they do not consider how air is needed to replace the volume of fuel being consumed, which directly relates to the pressure changes within the tank during fuel usage.