Which key do I use to zoom with Minecraft Optifine?

Default key: Ctrl (Although in some versions it's c)

If that doesn't work look for the option under Options > Controls > Zoom. Now you can see what key is bound for zooming. If you want to rebind it, click on the key next to the "Zoom" and press the key you want to rebind it to.

Actually, for most new optifine users, in 1.8 the default key for running is in fact Ctrl and optifine therefore makes the default key for zoom C. This is just if CTRL is not working :).

It's just C, or if it dosen't, work try Ctrl. If it still doesn't work, go to control settings and then to miscellaneous. Find zoom and change it to the key you want.