The correct option is D c
The option that has an error, is the correct option.
Option C: The error lies in part (c) as 'were' does not agree with the tense of the sentence. The words 'unless' and 'by next Monday' suggest that the action is a conditional one in the future. 'Were' refers to the past.
Hence, option C is the correct option with the error.
The other parts (a) and (b) do not have any error in them. They are grammatically correct and are complete in meaning.
Hence options A and B are not the right answers.
Option D is incorrect because the sentence has an error (c).