Which of the following is a common feature of all protists?

• A. Contractile vacuole. 
• B. Ocelloid.
• C. Flagellum.
• D. Chloroplasts.
• E. Pellicles. 
• F. None of the above.

Answer: (F)

Verified step by step guidance

1. Begin by understanding what protists are: Protists are a diverse group of eukaryotic microorganisms, which means they have a nucleus and other organelles enclosed within membranes.
2. Recognize that protists can be very different from one another. They can be plant-like, animal-like, or fungus-like, and they inhabit various environments.
3. Identify the features listed in the problem: Contractile vacuole, ocelloid, flagellum, chloroplasts, and pellicles. These are structures or organelles found in some protists but not all.
4. Understand that no single feature is common to all protists. For example, chloroplasts are found in plant-like protists (algae), flagella are found in some animal-like protists, and contractile vacuoles are found in some protists to help regulate water balance.
5. Conclude that the diversity among protists means that none of the listed features is universally present in all protists, which is why the correct answer is 'None of the above.'