For the same velocity λ = h/(mv), so the particle with smallest mass has the largest wavelength; a β-particle (electron) has much smaller mass than proton, neutron or α-particle, so option D is correct.

Bohr's condition quantizes orbital angular momentum as integer multiples of h/2π.

Compton scattering provided evidence for the particle-like behavior of photons (particle nature of radiation).

Hydrogen discharge produces a line (discrete) spectrum, so both 'line spectrum' and 'discrete spectrum' apply; the OCR shows option D selected.

Two half-lives (4 days) reduce A to 12 × (1/2)^2 = 3 g, so B = 12 − 3 = 9 g.

The book's answer key indicates option A; this corresponds to the given minimum wavelength per the provided answer listing.

Enrico Fermi led the development of the first controlled nuclear reactor, making him the correct answer.

Hydrogen has infinitely many energy levels, allowing infinitely many possible transitions and thus infinitely many spectral lines.

Bohr radius scales as n^2, so r4 = r2 × (4^2/2^2) = r2 × 4 = 2.12 Å × 4 = 8.48 Å.

Use photoelectric: eV_stop = hc/λ - hc/λ_threshold; plugging wavelengths gives the stopping potential ≈ 8.27 V.