The given sentence needs to be filled with the most suitable adjective. Looking at the sentence, we can say that it requires a superlative adjective ('I have ever read'). The words 'ever' and 'the' show the need for a superlative adjective. Hence the correct answer is A - most interesting - This is the most interesting novel I have every read. Option B is incorrect, as 'more interesting' is the comparative adjective, which isn't applicable here. Option C is incorrect, as 'less interesting' isn't an appropriate adjective here. Option D is incorrect as the correct answer is A.