After a long and arduous journey the explorers finally reached the summit.
Correct: A
Option A correctly places a comma after the introductory phrase 'After a long and arduous journey.' An introductory phrase or clause that precedes the main clause usually requires a comma to separate it. Options B and C misplace or incorrectly add commas. Option D uses a semicolon incorrectly; a semicolon is used to connect two independent clauses or to separate items in a complex list, not to separate an introductory phrase from the main clause.