Normally both using transport layer for communication but they are using tcp protocol which uses application layer so application layer is correct answer..
TELNET operates at Application Layer in TCP/IP Model. It's an Application to establish a connection to TCP Port Number 23, where a Telnet server application (telnetd) is listening.
encapsulation
application
Physical Layer, SONET is the other name used for it
Every layer add its relevant data, (required for various typical operations such as identify packet location, sender/ receiver addresses, App. ports, etc) along with actual data to transmit. Where as, every activities related to Packet is typically being performed in Network Layer/ IP layer.
TELNET operates at Application Layer in TCP/IP Model. It's an Application to establish a connection to TCP Port Number 23, where a Telnet server application (telnetd) is listening.
Telnet is an application layer protocol (Layer 7). Its functionality is similar to any of the layer 7 protocols, including DHCP, DNS, FTP, etc.
TELNET
TELNET Application layer services like Network Virtual Terminal are used to realise this. TELNET is the protocol used. Refer - Data Communication and Networking by Forouzan
Telnet is a Network layer protocol. It is used to create connection remotely.
http telnet
TCP port 23
Telnet is similar to HTTP and FTP in that all three use the TCP protocol for connection-oriented
FTP and Telnet are considered OSI layer 7 protocols. If you are blocking the service, then layer 3 or 4 (network or transport) is responsible.
You are probably asking about telnet protocol, which is a layer 7 application layer protocol.
Telnet
The firewall blocks it. Almost all router have telnet port blocked because it's considered dangerous.