Which of the following isotopes would most likely be unstable and therefore radioactive?
helium-3
calcium-40
molybdenum-98
mercury-194

Mercury-194 is an unstable isotope and hence is radioactive.

Explanation: Mercury-194 is an isotope of mercury, having formula

Number of protons in this isotope = 80

Number of neutrons in this isotope = 114

This isotope is radioactive in nature and under decay process by Electron Capture.

Electron capture reactions are the reactions in which a proton in a nucleus absorbs an electron and convert it into neutron. The resulting nucleus will have a decreased atomic number and same atomic mass.

Reaction for electron capture of mercury-194 follows:

the volume increases