Final answer:
Stars cross the meridian at an altitude equal to their declination minus the observer's latitude, which is option C.
Step-by-step explanation:
Stars cross the meridian at an altitude equal to their declination minus your latitude, which corresponds to option C. This altitude can be determined by observing the angle between the direction of a star and the zenith (the point directly overhead). At the point of crossing the meridian, this angle is the declination of the star minus the observer's latitude. For example, if a star has a declination of 40° and you are at a latitude of 30°, the star will cross the meridian at an altitude of 10° above the horizon. This is because the declination is the angle north or south of the celestial equator and latitude is the angle north or south of the Earth's equator, so when a star is at the meridian, its altitude is the difference between these two angles.