To answer this question, you need to have knowledge of code-behind classes and their purpose.
The code-behind class is a concept used in web development, specifically in the ASP.NET framework. It is a file that contains the server-side code for a web page or user control.
Server-side code, as the name implies, runs on the server and is responsible for processing requests and generating the HTML that is sent to the client's browser. The code-behind class typically handles events and performs data processing and manipulation.
Client-side code, on the other hand, runs in the client's browser and is responsible for manipulating the HTML elements and providing interactivity. Examples of client-side code include JavaScript and CSS.
Based on this information, we can conclude that the type of code found in the code-behind class is server-side code. Therefore, the correct answer is A) Server-side code.