Final answer:
A test case is a comprehensive scenario in software testing, including initial conditions, test actions, and expected outcomes, to validate a system's functionality.
Step-by-step explanation:
A test case, as defined in option B, is 'A set of preconditions, inputs, actions, expected results, and post-conditions, developed based on test conditions.' This definition encapsulates the comprehensive nature of a test case in software testing or systems engineering. It is not merely a single input or output but a scenario including the environment set-up (preconditions), the tests to be executed (inputs and actions), what outcome is expected (expected results), and the state after the test has been conducted (postconditions).