The nucleus of unstable of an element will decay leading to emission of radiation. question 1 options:

Isotopes of same element has different number of neutrons with different masses and having same number of protons and electrons.

Radioactive isotopes are those isotopes which are radioactive in nature. The unstable nucleus results in the radioactivity process and this process will go on until the stable isotope (element) forms.

Thus, the nucleus of unstable isotopes of an element will decay leading to emission of radiation.