It is ironic and somehow tragic that good people are often dull while evil people can be endlessly
The word "ironic" signifies a contradiction of ideas within the sentence, and the conjunction "while:" indicates that the characteristics of the good people and the evil people are being contrasted. Therefore, if the good people are often 'dull', the evil people must be having a characteristic which is the opposite of being 'dull'.
Among the given choices, it is only the word 'fascinating' that is opposite in meaning to the world 'dull'. So, D is the answer.