Which of the following statements is true regarding half-life?

The statement that it varies for each radioactive element is true. Half-life is a fundamental characteristic of radioactive decay that refers to the time it takes for half of a sample of a radioactive substance to decay into its daughter products. Each radioactive isotope has a distinct half-life, which can range from fractions of a second to billions of years. This variability arises from differences in the stability of the nuclei of different isotopes, as some are more prone to decay than others.

In contrast, the uniformity of half-life across isotopes would imply that all substances decay at the same rate, which is not the case. The notion that half-life indicates the time until total decay is also misleading because it only refers to the time taken for half of the original amount to decay, not the complete disappearance of the substance. Lastly, while half-life can indeed be measured in any consistent unit of time, the specific choice of units (seconds, minutes, hours, years, etc.) does not change the inherent property of half-life itself—it simply describes the same phenomenon in different time scales.