After a long exhausting day, the explorers finally reached their destination a secluded, crystal-clear lake surrounded by towering mountains.
Correct: A
The sentence begins with an introductory phrase 'After a long, exhausting day'. 'Long' and 'exhausting' are coordinate adjectives modifying 'day', so a comma is needed between them. After this introductory phrase, a comma is required. The phrase 'a secluded, crystal-clear lake...' provides an explanation or elaboration of 'destination', making a dash an appropriate choice to introduce this appositive phrase for emphasis. Option A correctly places a comma between the coordinate adjectives, a comma after the introductory phrase, and a dash to introduce the explanatory phrase.