The correct Active/Passive form of the given sentence is:

☀ The employees who had not completed their project within stipulated time were given a warning by the CEO.

So, the correct passive form is: "The employees who had not completed their project within stipulated time were given a warning by the CEO."

The other options and their explanations:

☀ The employees were given a warning by the CEO who had not completed his project within stipulated time.

• This option changes the meaning of the original sentence. It suggests that the CEO, not the employees, is the one who had not completed a project within the stipulated time.

☀ The employees who had not completed their project within stipulated time was given a warning by the CEO.

• This option contains a subject-verb agreement error. The subject "employees" is plural, so the correct form should be "were given" instead of "was given."

☀ The employees who had not completed their project within stipulated time were being given a warning by the CEO.

• This option is in the passive voice, but it uses the past continuous tense ("were being given") which suggests an ongoing action in the past, rather than the simple past tense used in the original sentence. The correct passive form uses the simple past tense "were given."