which cable do I need to connect usb-c 2021 M1 IPAD pro to iMac (Retina 5K, 27-inch, Late 2014)

Since your iMac is so old (no offense intended) and it only has Thunderbolt 2 ports, you will need a new cable. iPad Pro M1 supports Thunderbolt 3 only. I suggest using a USB-C to USB cable that you can buy on Amazon-

https://www.amazon.com/AmazonBasics-Type-C-USB-Male-Cable/dp/B01GGKYLW0/ref=sr_1_1_sspa?dchild=1&keywords=USB-c+to+usb&qid=1631031774&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzN0FTOURKRFVIVVlTJmVuY3J5cHRlZElkPUEwNDIzMzk2MjJRODZLVzlHSE1ZSyZlbmNyeXB0ZWRBZElkPUEwNTg2OTAxMzQzOTRLR0JLVVkzUCZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

I hope this helps. Have a nice day.