The correct answer is: A. Mean sun
The hour angle of a heavenly body is the angle between the meridian of the observer and the meridian of the body, measured eastward from the observer’s meridian along the celestial equator. The hour angle of the mean sun for Greenwich meridian equals the hour angle of the body for any other meridian plus longitude.
The mean sun is a fictitious sun that moves across the sky at a constant rate. It is used to define the mean solar day, which is the average time it takes for the Earth to rotate once on its axis. The mean solar day is 24 hours long, but it is not exactly 24 hours long because the Earth’s rotation is slowing down very slowly.
The true sun is the actual sun. It moves across the sky at a variable rate because the Earth’s orbit around the Sun is not a perfect circle. The true solar day is therefore slightly longer than the mean solar day.
The vernal equinox is the first day of spring in the Northern Hemisphere. It occurs when the Sun crosses the equator from south to north. The vernal equinox is not relevant to the question of the hour angle of a heavenly body.