Answer
(a) an electron
Work Step by Step
de-Broglie wavelength is inversely proportional to the product of mass and speed of particle.
$\lambda= \frac{h}{mv}$ (section 28.2)
so for the same speed lighter particle will have longest de Broglie wavelength.
Among the given options electron is lightest so it will have longest de Broglie wavelength.
