In web applications, what do we call the small text file stored on the client machine, either on disk or in browser memory, that allows the server to persist key value data across requests?

Introduction / Context:
Web applications use several mechanisms to maintain state and store data between requests. One of the oldest and most widely used mechanisms is the cookie. Cookies are small pieces of text that allow servers and clients to remember information such as login tokens, preferences, and tracking identifiers. This question checks whether you can distinguish cookies from other state management features such as view state and query strings.

Given Data / Assumptions:

• We are dealing with HTTP based web applications.
• The data is stored in a small text file or in memory on the client side.
• The server uses this data across multiple requests to maintain state.

Concept / Approach:
Cookies are name value pairs stored by the browser on behalf of websites. When the server sends a Set Cookie header, the browser saves the cookie in a text file or appropriate storage associated with the domain. On subsequent requests to that domain, the browser automatically includes the cookie values in the Cookie header. This mechanism allows servers to associate requests with user specific data without embedding all state directly in URLs or forms.

Step-by-Step Solution:
Step 1: A server responds to an HTTP request with headers that may include one or more Set-Cookie headers; each header defines a cookie name, value, and attributes such as expiration and path. Step 2: The browser stores these cookies in its cookie store, which may be backed by small text files or a database like storage. Step 3: When the user makes another request to the same domain and path, the browser automatically sends the Cookie header containing relevant cookies back to the server. Step 4: The server reads the cookie values and uses them to identify a session, remember user preferences, or perform analytics and tracking. Step 5: Cookies can be persistent, with an expiration date, or session based, which means they exist only for the lifetime of the browser session.

Verification / Alternative check:
You can verify cookie behaviour using browser developer tools. Under the Storage or Application tab, you can inspect cookies stored for each domain, including their names, values, and expiration times. Network tools show Set-Cookie and Cookie headers in HTTP requests and responses. This confirms that cookies are the mechanism described, not view state or query strings.

Why Other Options Are Wrong:
Option A is wrong because view state is a hidden field mechanism in ASP.NET Web Forms that stores page state inside the page itself, not as a separate file on the client. Option C is incorrect because query strings attach data to URLs as part of the request line, not as locally stored files that persist across sessions. Option D is misleading because session state is typically stored on the server or in dedicated providers, not as plain text files directly controlled by the browser.

Common Pitfalls:
Common pitfalls include storing sensitive information such as passwords in cookies without proper encryption, and forgetting to mark cookies as secure or HTTP only when appropriate. Another issue is relying heavily on large cookies, which can bloat request headers and affect performance. Understanding how cookies work helps developers design secure and efficient state management strategies that comply with privacy and security best practices.

Final Answer:
The small client side text file described is called a cookie, which stores key value data on the client so that the browser can send it back to the server on subsequent requests.