Altitude is best defined as a:

Altitude is best defined as the position of an object above or below sea level. This definition is grounded in the concept of elevation relative to a baseline, which is typically the average level of the ocean. Altitude is crucial in various fields, including aviation, meteorology, and mountaineering, where understanding how high something is compared to sea level can impact performance and safety.

In aviation, for instance, pilots need to be aware of their altitude to ensure they are flying at safe levels above terrain and obstacles. In the context of firefighting or emergency response, knowing the altitude can influence decisions about water supply, equipment operation, and personnel safety, especially in areas with significant elevation changes.

The other choices do not accurately define altitude. Measurement of temperature pertains to thermal dynamics and is not related to the vertical position of an object. The angle of elevation from the horizon relates to angular measurement rather than the vertical distance from a reference point. Lastly, the distance from the Earth's core is a radial measure and does not indicate altitude, which specifically pertains to an object's position in relation to sea level.