Final answer:
The correct option that is NOT true about cyclins is that they are not marked with ubiquitin for degradation.
Step-by-step explanation:
The correct option that is NOT true about cyclins is C. ubiquitin mark targets them for degradation. Cyclins bind to Cdk's to help activate them (option A), have no intrinsic enzymatic activity (option B), and their concentration varies during the cell cycle (option D). However, cyclins themselves are not marked with ubiquitin for degradation.