What's meant by a diurnal rhythm ? And accordingly, How can I solve the following physiology MCQ?

“Diurnal” is not a medical term. It’s just an English word. The opposite of “nocturnal.”

https://www.dictionary.com/browse/diurnal

In physiology I believe a diurnal rhythm means a consistent daily rhythm. But you would know better than I. I think the answer is ACTH but don’t just take my word for it.

I think it’s ACTH too.
“To test ACTH levels, healthcare providers may require you to fast overnight and conduct the test early in the morning. This is because ACTH levels are high in the morning and gradually decrease during the day. ACTH is at its lowest level during sleep.”

https://www.hormone.org/your-health-and-hormones/glands-and-hormones-a-to-z/hormones/adrenocorticotropic-hormone-acth