Option D: In the given sentence, two people are spoken of: Mark Twain and William Shakespeare. It's the subject of the sentence. The verb that follows 'are' is plural and agrees with the number of the subject. The phrase 'a' highly appreciated author' speaks of a singular noun, whereas the subject has two. Hence the underlined part must be changed and option D is wrong.
Option A: 'Well-regarded writers' refers to both people in the subject. Hence it's the most suitable option.
Option B: 'Highly appreciated writer' also refers to a single writer, but the sentence has two names mentioned. Hence B is wrong.
Option C: Similarly, 'well known speaker' also refers to a singular noun and hence C is also wrong.