The correct answer is C) points where there is no variation. The agonic line on an isomagnetic chart indicates the points where there is no magnetic variation, meaning the compass direction precisely matches true north. This is because the agonic line represents the area where the magnetic and geographic north poles are aligned, resulting in zero variation between magnetic and true north. The other answer choices are incorrect because they do not accurately describe the function of the agonic line. Option A refers to a magnetic longitude reference line, which is different from the agonic line. Option B describes points of no annual change in variation, which is not the same as no variation. Option D describes the magnetic equator, which is a different feature on the chart.