Answer
$Beta$ decay.
Work Step by Step
The Molybdenum decay to Technetium by $Beta$ decay. This is because the Technetium is the isobar produced from the Molybdenum atom releasing electron and electron antineutrino. Here the lepton number is conserved according to the decay equation,
$^{99}_{42}Mo \longrightarrow ^{99}_{43} Tc + e^- + \overline{v}$
