The correct option is A a
The option that has an error, is the correct option.
Option A: The error lies in part (a): 'Mutton is more hard'. The words 'more hard' are erroneous.
The comparative form of 'hard' is 'harder'. We use the words 'more' and 'most' for those adjectives (e.g. beautiful, more beautiful, most beautiful) that cannot form comparatives with -er and superlatives -est (e.g. hard, harder, hardest).
The correction is 'Mutton is harder'.
Hence option A is the correct option with the error.
The other parts (b) and (c) do not have any error in them. They are grammatically correct and are complete in meaning.
Hence options B and C are not the right answers.
Option D is incorrect because the sentence has an error (a).