Hello Mohamed
Even though it is used to exchange routing and reachability information, which is a Layer 3 functionality, BGP itself does indeed operate at the Application layer. Because it uses TCP to actually exchange information, it is the Application layer entity of BGP that provides the data for the TCP segments. So BGP is indeed an Application layer protocol.
Compare this with OPSF. It doesn’t use any functionalities above Layer 3, so OSPF is only a Layer 3 protocol.
I hope this has been helpful!
Laz