The correct answer is option 2: chemiluminescence process.

Option 1 states that the light emitted by fireflies is due to a radioactive substance. This is incorrect. The light emitted by fireflies is not a result of radioactive decay.

Option 3 suggests that the light emitted by fireflies is due to a photoelectric process. While some organisms, such as plants, use a photoelectric process to convert light energy into chemical energy, fireflies do not use this process to emit light.

Option 4 states that the light emitted by fireflies is due to burning of phosphorus. This is incorrect. Fireflies do not produce light through a combustion reaction.

The correct answer, option 2, explains that fireflies emit light through a process called chemiluminescence. This process involves a chemical reaction within the firefly`s body that produces light without any significant heat. Fireflies have special organs in their abdomens that contain luciferin, a pigment, and luciferase, an enzyme. When luciferin reacts with oxygen in the presence of luciferase and ATP (adenosine triphosphate), light is produced. This is what causes fireflies to glow.