The correct answer is option 1: Gamma rays have zero mass and no charge.

Gamma rays are a form of electromagnetic radiation, just like visible light or X-rays. They are produced during certain nuclear reactions or radioactive decay.

Mass refers to the amount of matter in an object, and gamma rays have zero mass because they are not made up of particles with mass, such as protons or electrons. Instead, they are pure energy.

Charge refers to the electric charge of an object, either positive or negative. Gamma rays, being neutral, do not have any charge.

Option 2, which suggests that gamma rays have a positive charge, is incorrect. Option 3, which suggests that gamma rays have unit mass and zero charge, is also incorrect. Lastly, option 4, which suggests that gamma rays have unit mass and a negative charge, is also incorrect.

Overall, the correct answer is option 1: Gamma rays have zero mass and no charge.