Which part of the following sentence contains an error? Both Raghunath as well as Ravish (a) have given (b) their consent (c) to the new proposal (d)

The error is in part (a). The construction 'Both...as well as' is incorrect; the correct correlative pairing is 'Both...and' or 'as well as' alone (without 'both'). The sentence should read 'Both Raghunath and Ravish' or 'Raghunath as well as Ravish'. So part (a) contains the error.