This is how Alexa Donne (a wonderful Authortuber) recommended you reply:
THE OFFER OF REPRESENTATION NUDGE
Subject line: add OFFER OF REPRESENTATION or (Offer of rep) to the subject line of your original email chain, or if an agent requests follow-ups/offers of rep go to a new email address, start a fresh email with OFFER OF REPRESENTATION: BOOK TITLE (your name), and then I would c&p your original query below your signature to refresh them.
Dear [Agent's name],
I wanted to let you know that I've received an offer from an agent. I have not yet responded to the offer, but as a courtesy to the offering agent, I would love to do so by DATE 10 DAYS TO 2 WEEKS IN THE FUTURE.
Scenario 1: Thank you for your consideration, and I look forward to hearing from you.
Scenario 2: The agent offered on a revised version of the manuscript, different from the one I previously sent to you. I've attached that version, in case you would prefer to read it.
I look forward to hearing from you.