Final answer:
The regulator points in question are opened by an increase in electromagnetic field strength, which is achieved by increasing the current flowing through the coil of the regulator. Hence, the correct answer is an increase in current.
Step-by-step explanation:
The question relates to the operation of a voltage regulator in an electromagnetic context. Specifically, it asks what would cause the points in a voltage regulator to open at a voltage of 14.25V. The opening of these points is typically triggered by an increase in electromagnetic field strength within the regulator. In turn, this increase in field strength can be caused by an increase in current flowing through the coil of the regulator. Therefore, the correct answer to the question is C) Increase in current, Increasing the current in an electromagnetic coil will strengthen the magnetic field up to a point where it can mechanically open the points of a voltage regulator. The voltage threshold mentioned (14.25V) acts as a reference point at which the system is balanced and the current level would be sufficient to affect the regulator's points.