The Application Layer determines the identity of the communication partners and this is where Non-Repudiation service would be provided as well. See the layers below:

DOD Model DoD Model
The following answers are incorrect:
network layer. Is incorrect because the Network Layer mostly has routing protocols, ICMP, IP, and IPSEC. It it not a layer in the DoD Model. It is called the Internet Layer within the DoD model.
transport layer. Is incorrect because the Transport layer provides transparent transfer of data between end users. This is called Host-to-Host on the DoD model but sometimes some books will call it Transport as well on the DoD model.
data link layer. Is incorrect because the Data Link Layer defines the protocols that computers must follow to access the network for transmitting and receiving messages. It is part of the OSI Model. This does not exist on the DoD model, it is called the Link Layer on the DoD model.