Alpha-particles are

(a) Protons

(b) Positron

(c) Neutrally charged

(d) Ionized helium atoms

(d)

We know that alpha particles are the nucleus of ionized helium atoms which contain two protons and two neutrons. These are emitted by the'' nuclei of certain radioactive substances. Streams of alpha particles, called a-rays, produce intense ionisation in gases through which they pass and are easily absorbed by matter.