The correct option is (d) β - particle.
Explanation:
de Broglie wavelength for a particle is given by λ = h/p = h/mv, where m, v and p are the mass, velocity and momentum respectively. it is Planck'* constant. Now, since all the particles are moving with same velocity, the particle with least mass will have maximum de-Broglie wavelength. Out of the given four particles (proton, α -particles, i.e. He nucleus and β-particles. i.e. electrons) β-particles has the lowest mass and therefore it has maximum wavelength.
