If particles are moving with same velocity, then maximum de-Broglie wavelength will be for

$\mathrm{\lambda }=\frac{\mathrm{h}}{\mathrm{mv}}$
Where m, v are the mass and velocity. All the particles are moving with same velocity, the particle with least mass will have maximum de-Broglie wavelength. $\mathrm{\beta }$ particles has the lowest mass and therefore it has maximum wavelength.