Final answer:
Altitude means 'the height above a surface,' which in geographical terms is usually related to the height above sea level. The higher the altitude, the lower the air density and temperature, as seen at high elevations like Mount Everest.
Step-by-step explanation:
In the sentence provided, 'At that altitude, it is extremely cold, and there is almost no air,' the word altitude refers to option B, 'the height above a surface.' Altitude is a measure of how high something is in relation to sea level or the earth's surface. For example, the Figure A4 Altitude-Air Density Relationship you mention shows that at the altitude of 8,828 meters, which is the height of Mount Everest, the air density is extremely low (0.023 kilograms per cubic meter). This relationship is notably important because as altitude increases, the air becomes thinner, which means air density decreases, making it colder and harder to breathe.