On heating ${ }^{24}\mathrm{Na}$ in a flame, its half-life period of 15h

On heating ${ }^{24}\mathrm{Na}$ in a flame, its half-life period of 15h is remains unchanged.

A substance's half-life is the amount of time it takes for half of it to decompose. When referring to radioactive elements, where the number of atoms gradually decreases over time by converting into new atoms, the term "half-life" was originally employed.

Hence, option D is correct.