How to refuse an offer politely without ruining the relation with the placement consultant?

There's nothing to worry about declining an offer. You can politely mention that you had several opportunities in hand. Based on all parameters and your personal goals, another opportunity is better suited to you at this point of time. Then, mention that you wouldn't be able to move forward with their opportunity and wish them good luck with finding another person. If you like, you can mention some positive aspects of their process like swift and smooth process, hiring manager's support during the process etc. In case, you know someone looking for a similar role, referring them will be a great gesture when you can't take this role. Know that they will surely try to ask you more questions like what is the other opportunity, can we match the salary etc. But, you do not have to tell more than the above. It is sufficient to say you got another opportunity that match your goals better. It is similar to how companies share the message to rejected candidates about moving forward with another candidate that matches their requirements better.

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