Step 1: Understanding tense usage.
The sentence describes an action that was already completed before another action ("they reached"). For such cases, we use the Past Perfect tense.
Step 2: Analyzing the options.
(A) rings – Simple Present, incorrect for past context.
(B) had rung – Correct, as it denotes an action completed before another past event.
(C) rang – Simple Past, does not convey the "before another past action" meaning.
(D) will ring – Future tense, incorrect.
Step 3: Conclusion.
The correct answer is (B) had rung.