To answer this question, the user needs to know the basic components of a web application and their respective roles.

A web application is built using a combination of client-side and server-side technologies, where the client-side is the part of the application that runs on the user's device (such as a web browser), and the server-side is the part that runs on a web server.

Given the options:

A. Web client: This option is incorrect because a web client is the device or software that accesses a web application, such as a web browser. The web application itself does not reside on the web client.

B. Web server: This option is correct. A web server is where a web application resides. The web server hosts the server-side code and serves the application to client devices upon request.

C. Visual Studio .NET: This option is incorrect because Visual Studio .NET is a software development environment and is not related to the deployment of web applications.

D. Both a and b: This option is partially correct, but not complete. While a web client is necessary to access a web application, the application itself does not reside on the client device. The web server is the place where a web application resides and where it is hosted.

E. All of the above: This option is incorrect because Visual Studio .NET is not related to the deployment of web applications. The correct answer is option B.

Therefore, the answer is: B) Web server.