(A) Typical internet chat is a client/server app, since a client makes a connection to a central server and receives update. There are some chat systems that work on a P2P model.
(B) Web browsing is client/server, where a browser is the client and the web page is served up by a server.
(C) E-mail is typically thought of as a client/server app, although hops will be made between POP servers.
(D) Ping uses ICMP protocol, and a ping client sends a request to a destination and gets back a response. It is a connection testing tool.
So, D is the answer