Which part of the following sentence has an error in it? 'Once you have made this decision (a) there would be (b) no going back' (c), Ram warned me (d).

Part (b) contains the error. After 'Once you have made this decision' (present perfect), the correct construction should be 'there will be no going back' (future tense) rather than 'there would be' (conditional). The conditional 'would' does not match the present perfect tense in the first clause. The sequence of tenses requires 'will' in this context.