Need SSD drive, which is the correct one?

Your system has two ports for drives! A SATA connection to support either a 3.5” HDD or 2.5” SSD drive. As well as a custom blade SSD port which is on the backside of the logic board.

Here’s the needed kit for replacing your current HDD for a SSD iMac Intel 27" (Late 2012-2019) SSD Upgrade Kit If your system did not come with a HDD you’ll need a SATA cable Intel iMac 27" Late 2013 Hard Drive Cable

The other choice is to install the much faster blade SSD following this guide iMac Intel 27" EMC 2639 Blade SSD Replacement iFixit doesn’t offer the blade SSD’s. Review this great guide The Ultimate Guide to Apple’s Proprietary SSDs Jump down to Generation 3 section.