Which of the following elements is commonly known for its isotopes?

Hydrogen, oxygen, and carbon are all elements that have well-known isotopes, making "all of the above" the most accurate answer.

Hydrogen has three primary isotopes: protium, deuterium, and tritium, which differ in the number of neutrons in their nuclei. This diversity in isotopes contributes to various applications, including their use in nuclear fusion and as tracers in biochemical research.

Oxygen has three stable isotopes: oxygen-16, oxygen-17, and oxygen-18. The variations in these isotopes can influence analyses in areas like climate science and paleoclimatology, where oxygen isotope ratios help determine historical temperature changes.

Carbon is another element with important isotopes, the most famous being carbon-12 and carbon-14. Carbon-14 is particularly known for its application in radiocarbon dating, a technique used to determine the age of organic materials.

Thus, since each of these elements has isotopes and applications tied to them, choosing "all of the above" reflects the fact that they are indeed all commonly recognized for their isotopes.