* Half-life is an intrinsic property of a radioactive isotope. It's determined by the specific nucleus of the atom and how unstable it is. Different isotopes of the same element will have different half-lives.

Let's look at why the other options are incorrect:

* A) Mass: The mass of a sample doesn't affect its half-life. A larger sample will have more atoms, but each atom decays at the same rate.

* B) Temperature: Temperature changes can affect the rate of chemical reactions, but they don't significantly alter the rate of radioactive decay. Radioactive decay is a nuclear process, governed by the weak force, and is not affected by external temperature fluctuations.

* C) Addition of a catalyst: Catalysts speed up chemical reactions by providing an alternative reaction pathway. Radioactive decay is not a chemical reaction; it's a nuclear process, and catalysts have no effect on it.

In summary: The only factor that affects the half-life of a radioactive substance is the type of radioactive isotope.