Which of the following elements is NOT a radioactive source used in nuclear gauges?

Hydrogen is not a radioactive source used in nuclear gauges, primarily because it is a stable element with no isotopes that exhibit radioactivity. In contrast, the other elements listed—Californium, Americium, and Cobalt—are known for their radioactive isotopes that are utilized in various applications, including nuclear gauges.

Californium, for example, has isotopes that are used in neutron sources for medical and industrial applications. Americium is commonly found in smoke detectors and certain types of nuclear gauges due to its use in the form of Americium-241. Cobalt, specifically Cobalt-60, is also a radioactive isotope often used in radiation therapy and in industrial radiography.

Thus, given the context of the question, hydrogen stands out as the correct answer, as it does not have any radioactive qualities relevant to the usage in nuclear gauges.