The correct option is B b
The option that has an error, is the correct option.
The sentence refers to a past event that happened 'last seminar'.
Option B: The error lies in part (b): 'send their papers'. The word 'send' is the error.
'Send' describes an action in the simple present tense. But, the adverb 'last seminar' tells us that the action has already taken place in the past.
The correction is 'sent their papers'. The verb 'sent' is a past tense verb.
Hence option B is the correct option with the error.
The other parts (a) and (c) do not have any error in them. They are grammatically correct and are complete in meaning.
Hence options A and C are not the right answers.
Option D is incorrect because the sentence has an error (b).