Answer:
The correct answer is (b) a colon.
Many who have attempted to climb Mount Everest have sacrificed their most valuable possession to the mountain: their lives.
Step-by-step explanation:
A colon is used in a sentence in order to amplify the effect of the information provided in the part of the sentence that precedes the colon by the information that follows it.
In the above sentence, the author intends to amplify the effect of information that many of those people who attempted to climb Mont Everest had to lose their lives. S/He does so by not writing the sentence directly like 'Many who have attempted to climb Mount Everest have sacrificed their lives to the mountain'. Instead, the author has broken up the sentence into two parts separated by a colon to achieve the desired effect. Thus, the correct formation of the sentence should be "Many who have attempted to climb Mount Everest have sacrificed their most valuable possession to the mountain: their lives."