Final answer:
The answer to the fill-in-the-blank question is 'elevation'. Elevation specifically refers to the height above sea level, which is applicable to Mount Washington's height in the White Mountains. Mount Everest also serves as an exemplary case, with its summit once being part of an ancient seabed.
Step-by-step explanation:
Mount Washington, which rises to an elevation of 6,288 feet, is the highest peak in the White Mountains. The correct word to fill in the blank is 'b) Elevation'. Elevation refers to the height of a point on the Earth's surface above sea level, while 'altitude' is often used more broadly to refer to the height of an object in the atmosphere. The 'apex' refers to the top or highest part of something, but it is not specific to geographical heights. Similarly, the 'summit' is the highest point of a mountain but does not specifically refer to the height above sea level.
In contrast, Mount Everest, the tallest mountain on Earth, has an elevation of 29,035 feet. At one time, it was part of an ancient seabed, as evidenced by the layer of unaltered limestone from the Ordovician at its summit. This indicates that the environment of Mt. Everest during the Ordovician period was marine.