The expedition detected five shipwrecks in the bay,
which triples to fifteen the number of wreck sites now known as littering the bay's floor.
Here, the intended meaning is - the expedition (S) detected (V) five shipwrecks in the bay and this action simultaneously increased the number of wreck sites...
Noun, which is essentially a noun modifier modifying a very nearby noun -
but we need an adverbial modifier to modify the action 'detected' - A, C and E are incorrect
Noun + Verb +....., verb-ing is essentially an adverbial modifier modifying an action - So compare option B and D
(B) tripling to fifteen the number of wreck sites now
known to litter
(D) tripling to fifteen the number of wreck sites now
known as littering
On comparison of B and D, known to is the correct usage
'known as' - Usage already brought out by
zhanbo above
so D is incorrect
Answer: B