A photon has 8.0 × 10–15 J of energy. Planck’s constant is 6.63 × 10–34 J•s.

Use the photon-energy relationship E = hf, so the frequency is f = E/h. Substituting gives f = (8.0 × 10⁻¹⁵ J)/(6.63 × 10⁻³⁴ J·s) ≈ 1.21 × 10¹⁹ Hz. Therefore, the correct choice is A, 1.21 × 10¹⁹ Hz.

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