Debugging can be enabled in an Express application for development purposes by using the DEBUG environment variable. This allows you to selectively enable debugging for specific modules or components. The other options do not enable debugging in the standard way for Express applications.

Cache coherence in a distributed caching system is typically maintained using a consistency protocol like "invalidation." This protocol ensures that cached data is invalidated or updated when changes occur, ensuring data consistency across distributed nodes.

To secure sensitive information like API keys in an Express application, it is essential to use environment variables. Storing API keys in plain text within your code is not secure, embedding them in URL parameters can expose them, and including them in client-side JavaScript makes them accessible to anyone inspecting the client code.

In Express, the app.get() method is used to handle HTTP GET requests to a specified route. This method is a part of the Express application instance (app) and is used to define routes that respond to HTTP GET requests. The other options (app.post(), app.route(), and app.put()) are used for handling other HTTP methods (POST, routing, and PUT) respectively.

Client-side caching involves storing cache data in the client's browser to reduce server load and increase load speed. This can be achieved using techniques like browser caching and local storage.

The ?: operator is known as the ternary operator in JavaScript and is used for conditional operations. It allows you to perform a quick inline conditional check and return one of two values based on the condition. The other options (if-else, switch, for) are not used for conditional operations in the same way.

To validate multiple conditions effectively and execute specific code blocks for filtering out invalid user inputs, you would typically use if...else if...else statements. These conditional structures allow you to check various conditions and perform different actions based on the outcome. The switch statement is more suitable for situations with a single value and multiple cases, and try...catch is primarily used for error handling. A while loop is not a conditional structure for validation but a looping mechanism.

When you install packages using npm, the dependencies are listed in the "package.json" file under the "dependencies" section. This file acts as a manifest for your project and keeps track of its dependencies.

In Jest, the toMatchSnapshot function is used to generate snapshot files of a component's output render. This allows you to capture the expected output and compare it to the actual output during subsequent test runs. The other options do not represent the correct Jest function for this purpose.

In JavaScript, a variable declared using the var keyword inside a function has function scope. This means it is accessible within the entire function but not outside of it. Unlike let and const, var does not have block scope.