The radioactive emission with the least ionization power is

Correct answer: γ-rays

Explanation: Ionizing power decreases in the order α > β > γ (and X-rays). Alpha particles are heavy and highly charged, producing many ion pairs; beta particles are lighter and cause moderate ionization. Gamma rays are neutral, highly penetrating photons and produce the fewest ions per unit path, so they have the least ionization power.

• α-particles — Highest ionizing power (wrong).
• X-rays — Low ionization, but generally more than γ for typical energies (not the least).
• γ-rays — Correct. Least ionizing, most penetrating.
• β-particles — Moderate ionization (wrong).