The correct answer is: it depends
Everest rises 8849.86 meters above sea level and is considered the highest mountain on Earth.
So if the question were:
“Is Everest the highest mountain above sea level?”
The answer is yes.
However, since it is not specified in the question, to be precise, there is an even higher peak, only it is partly located below sea level.
This is Mauna Kea, an ancient inactive volcano on the Hawaiian Islands. Considering the part submerged in the waters of the Pacific Ocean, in fact, Mauna Kea has a height of 9754 meters.