To use a custom exception, you can create an instance of the custom exception class and use the throw keyword to throw the exception. You can also optionally provide a custom error message or a chained exception cause as arguments to the custom exception constructor. The syntax for throwing a custom exception is:
throw new CustomException(optional_message, optional_cause);
Therefore, the options B and D are valid ways to fire the custom exception in Apex. The options A and C are invalid because they only create an instance of the custom exception class, but do not throw it. Also, the option C uses single quotes instead of double quotes for the error message, which is not allowed in Apex. References:
You can find more information about custom exceptions in the Apex Developer Guide and the Apex Reference Guide on Salesforce's official website. You can also read this blog post for a tutorial on creating and using custom exceptions.

When deploying to a production org, all Apex code, including triggers, must have at least 75% test coverage.
This means that at least 75% of the lines of code in each Apex class and trigger must be executed by unit tests.
This is a requirement to ensure the quality and reliability of the code and to prevent any unexpected behavior or errors in production. Test coverage is calculated by dividing the number of covered lines by the total number of lines in each class or trigger. Test coverage can be measured by running tests in the Developer Console, the Salesforce CLI, or the Apex Test Execution page in Setup. Test coverage can also be viewed in the Code Coverage page in Setup or in the Problems tab in the Developer Console. Test coverage is different from test results, which indicate whether the tests passed or failed based on the assertions and expected outcomes. Test results do not affect the deployment, but they can help identify and fix any bugs or issues in the code. Test coverage and test results are both important aspects of testing Apex code and ensuring its functionality and performance. References:
* Testing Apex from the Apex Developer Guide
* Get Started with Apex Unit Tests from the Salesforce Trailhead
* Deploying Apex to a Production Organization from the Apex Developer Guide