The correct option is A the largest
Option A: 'The largest' indicates the superlative degree of the sentence.
The given sentence is in superlative degree, because the whale is compared (in terms of size) with all other mammals.
('Of' in this sentence means 'out of', the whale is a part of the group 'living mammals', where the whale has the largest size.)
Hence option A is correct.
Option B: 'Largest' is an adjective of the superlative degree. Adjectives of this degree are mostly preceded by 'the'. Since the word is missing here, option B is incorrect.
Option C: 'The largest which is' is grammatically incorrect, because 'which' generally introduces a clause/phrase. But here, the words that follow 'of all living mammals' do not form a coherent phrase.
Option D: 'Larger than' is of the comparative degree. It compares two things, like 'the whale is larger than the shark'. But here, the comparison is between one thing 'whale' among a group 'of all living mammals'.
Hence, this option isn't correct.