Choose the most appropriate concluding statement for a formal letter from the options given below :- 

Looking for a quick reply

The most appropriate concluding statement for a formal letter is: Looking for a quick reply.

This statement is polite and professional, and it clearly conveys your request to the recipient. It also sets a clear expectation for when you would like to receive a response.

The other options are not as appropriate. Write to me is a bit too casual for a formal letter. Send someone here or else I will take action is threatening and could be considered rude. I am sick of writing is unprofessional and could damage your relationship with the recipient.