Comprehensive and Detailed Explanation From Exact Extract:

A valid test checks if the algorithm produces the expected output for a given input, according to its specification. The algorithm outputs "OK" for numbers in the range [98.3, 98.9] (inclusive or exclusive bounds not specified, but typically inclusive in such problems) and "Not OK" otherwise. According to foundational programming principles, we evaluate each test case.

Specification:

Input in [98.3, 98.9] → Output: "OK".

Input outside [98.3, 98.9] → Output: "Not OK".

Option A: "Input 98.6. Ensure output is 'Not OK.'" Incorrect. Since 98.6 is between 98.3 and 98.9 (98.3 ≤ 98.6 ≤ 98.9), the output should be "OK", not "Not OK". This test is invalid.

Option B: "Input 98.6. Ensure output is 'OK.'" Correct. 98.6 is within the range, and the expected output is "OK", making this a valid test.

Option C: "Input 99.9. Ensure output is 'OK.'" Incorrect. 99.9 is outside the range (99.9 > 98.9), so the output should be "Not OK", not "OK". This test is invalid.

Option D: "Input 99.9. Ensure output is '98.9.'" Incorrect. The algorithm outputs either "OK" or "Not OK", not a numerical value like "98.9". This test is invalid.

Certiport Scripting and Programming Foundations Study Guide (Section on Algorithm Testing).

General Programming Principles: Testing and Validation.

W3Schools: “Python Conditions” (https://www.w3schools.com/python/python_conditions.asp).