The ancient manuscript, which had been hidden for centuries, contained cryptic symbols therefore, deciphering it proved to be an arduous task.
Correct: B
The sentence contains two independent clauses: 'The ancient manuscript... contained cryptic symbols' and 'deciphering it proved to be an arduous task'. When two independent clauses are joined by a conjunctive adverb (like 'therefore', 'however', 'consequently', 'moreover'), a semicolon must precede the conjunctive adverb, and a comma must follow it. Option B correctly uses a semicolon before 'therefore' and a comma after it.