A valid test case consists of a set of inputs and the corresponding expected outputs based on the algorithm's specification. The algorithm's rule is: if the input is between 98.3 and 98.9, the output is "OK". The number 98.6 falls within this range. Therefore, a valid test is to provide 98.6 as input and verify that the output is "OK". This type of test, which checks the primary success scenario, is often called a "happy path" test.

A. This option contains a typographical error ("M98 9 ") for the expected output, making the test description invalid as written. A correct test would expect "Not OK".

C. The input 99.9 is outside the specified range. According to the algorithm's "else" condition, the output should be "Not OK", not "OK". This describes an incorrect expectation.

D. The input 98.6 is inside the specified range. The expected output should be "OK", not "Not OK". This describes an incorrect expectation.

1. Massachusetts Institute of Technology (MIT) OpenCourseWare. (2011). 6.170 Software Studio, Lecture 3: Testing. This lecture covers the principles of testing, including choosing test cases that cover different paths through the code, such as inputs that satisfy a condition and inputs that do not.

2. Ammann, P., & Offutt, J. (2016). Introduction to Software Testing (2nd ed.). Cambridge University Press. Chapter 1 defines a test case as including the input values and the expected results needed to evaluate the software.