The correct answer is Strontium-90.
Strontium-90 breaks down into yttrium-90.
Strontium-90 is a radioactive isotope of strontium. It undergoes β− decay into yttrium-90. A decay energy of 0.546 MeV is also released.
⁹⁰₃₈Sr -> ⁰₋₁e + ⁹⁰₃₉Yr
Beta particle has the charge and mass of an electron. Upon the release of a beta particle, the mass number of Sr remains unchanged but the atomic number of Sr increases by 1. As a result both Sr and Yt have mass number 90, however, Sr and Yt have atomic numbers 38 and 39 respectively.