What occurs during one half-life?

It's B) Half of a parent isotope undergoes radioactive decay to form a daughter isotope.

The half-life of a radioactive compound is the time taken for that said isotope to decay or disintegrate so that only half of the initial atoms remain in that compound. During the decay process, the isotope will give off energy and matter, and the way to depict this is indicated by t 1/2.