To determine whether sufficient testing has been performed, the user needs to know about the different types of testing and the processes involved in software testing.

Option A: Exit criteria are a set of conditions that must be met before a project or phase can be considered complete. This includes completing all the specified tests. Exit criteria can be used as a measure of whether sufficient testing has been performed. This option is correct.

Option B: Test Deletion Criteria refers to the conditions that determine when to remove tests from a test suite. It is not used to check whether sufficient testing has been performed. Therefore, this option is wrong.

Option C: A Test Plan outlines the testing approach, objectives, scope, and schedule for a testing project. It is not used to check whether sufficient testing has been performed. Therefore, this option is incorrect.

Option D: The Test Specification is a document that specifies the test requirements, test cases and test data for a software testing project. It is not used to check whether sufficient testing has been performed. Therefore, this option is wrong.

Therefore, the correct answer is:

The Answer is: A. Exit Criteria