I could not find one answer I truly like. After spending almost 3 minutes, I chose (D) but its punctuation use is questionable.
For irregular verb "ring", its past tense is "rang", and its past participle is "rung".
(A)The 16.5-ton bell known as “Great Paul”—rang traditionally at one o’clock each day—was the largest bell in the British Isles until the casting of the Olympic Bell for the 2012 London Olympics.
In the main clause, its subject is "The 16.5-ton bell", and its verb is "was the largest bell". Between the em-dash pair, a past participle phrase (rung...) should be used. Eliminate (A).
(B)The 16.5-ton bell known as “Great Paul”—traditionally rang at one o’clock each day—was the largest bell in the British Isles until casting the Olympic Bell
for the 2012 London Olympics.
See above. Eliminated.
(C) The 16.5-ton bell known as “Great Paul”—traditionally rung at one o’clock each day—was the largest bell in the British Isles until the Olympic Bell
had been cast for the 2012 London Olympics.
This option correctly uses past participle phrase "traditionally rung at one o’clock each day" between two em-dashes. We cannot choose (C), though, because past perfect tense is used in subordinate clause "until the Olympic Bell
had been cast for the 2012 London Olympics". The use of past perfect tense indicates that the Olympic Bell came into existence
before "Great Paul". Eliminate (C).
(D)The 16.5-ton bell known as “Great Paul”—traditionally rung at one o’clock each day
, was the largest bell in the British Isles until the Olympic Bell was cast for the 2012 London Olympics.
This option fixes the verb tense issue in (C), but uses a comma instead of em-dash to set off past participle phrase "traditionally rung at one o’clock each day". We should use em-dash in pairs.
(E) The 16.5-ton bell known as “Great Paul”—rang at one o’clock each day by tradition, and was the largest bell in the British Isles until the Olympic Bell was cast for the 2012 London Olympics.
Note the use of "rang" in this option. We might argue that “rang at one o’clock each day“ is the verb for subject "The 16.5-ton bell". But the use of em-dash is uncalled for.
I could not find a satisfactory option, but went for (D) in the end.