Final answer:
Calmodulin, which is a calcium-binding messenger protein, is activated by calcium ions and it modulates the activity of various enzymes.
Step-by-step explanation:
The correct answer to the question of which of these is activated by calcium ions is D) Calmodulin. Calcium ions have a pivotal role in cellular processes and function as a secondary messenger in many signaling pathways. The calcium ions, when released can bind to proteins like calmodulin, modifying their activity. In the context of enzymes, calcium ions can activate several, like ATPase, succinic dehydrogenase, and lipase; however, among the options given, calmodulin is the most appropriate as it is a calcium-binding messenger protein that becomes activated upon binding calcium ions which then modulates the activity of other proteins by interacting with various enzymes and influencing their activity.