Which is a correct way to enter an auto-update URL?

Which is a correct way to enter an auto-update URL?

A.
tftp://10.0.1.10/vpn3002-3.5.Rel-k9.bin

B.
http://10.0.1.10/vpn3002-3.5.rel-k9.bin

C.
http://10.0.1.10/vpn3002-3.5.Rel-k9.bin

D.
ftp://10.0.1.10/vpn3002-3.5.Rel-k9.bin