What is the latest operating system that runs on a MacBook Air 13 inch Early 2014?

Although Big Sur is the correct answer, that vintage Air came with either 4 or 8 GB RAM and up to a 512 GB solid-state drive. The integrated GPU is nearing 8 years old too. If this is a 4GB/128GB Air, I would in the strongest terms not recommend installing Big Sur on it. Too much operating system for the device, and it will crawl. Probably too much for an 8GB Air too as it is a huge operating system.

Big Sur.