In the client server model, what is the primary protocol used for communication between a browser and web server

Assuming the client is using a web browser, your answer would most likely be the Hyper-Text Transfer Protocol (HTTP).

HTTP is a very, very common way of transfering insensitive data from client to server. HTTPS is more secure, adding a secure socket layer to protect communications from being intercepted.

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As an alternative answer, the FTP (File Transfer Protocol) may be an acceptable answer, but this is usually not the primary method of communication for clients, as it is mainly used to access and modify files on the server, requiring a username-password combo to do so (usually).