If particles are moving with same velocity, then De-Broglie wavelength is maximum for : -

de Broglie wavelength: $\lambda = \dfrac{h}{mv}$
For the same velocity, $\lambda \propto \dfrac{1}{m}$; the lightest particle has the longest wavelength.
Masses: $\alpha$-particle $\approx$ 4 u, proton $\approx$ 1 u, neutron $\approx$ 1 u, $\beta$-particle (electron) $\approx \dfrac{1}{1837}$ u.
The $\beta$-particle (electron) has the smallest mass.
So the de Broglie wavelength is maximum for the $\beta$-particle.