Since 1868, approximately thirty-five shipwrecks have been recorded off Alaska, a region known for their treacherous

IMO D should be the answer


A. their treacherous seas—less than that........'their' referring to shipwrecks. We want something, which will refer to Alaska. Also, shipwrecks are countable. Wrong use of less.

B. its treacherous seas, a number fewer than that...............Tense issue.

C. its treacherous seas, which is less than those......Use of less for countable nouns. Also, IMO 'which' is referring to Seas

D. its treacherous seas—fewer than have been............correct tense

E. their treacherous seas, fewer than the ones..................'their' is referring to Shipwrecks.