Speed of light relates wavelength and period for EM waves in vacuum

Steps:

• Use formula c = λ / T, where c = 3 × 10^8 m/s for vacuum.
• Convert units: 1.2 μm = 1.2 × 10^{-6} m.
• Compute c for each option: λ / T.
• Verify only option C yields exactly 3 × 10^8 m/s.

Why C is correct:

• λ = 1.2 × 10^{-6} m and T = 4.0 × 10^{-15} s give c = (1.2 × 10^{-6}) / (4.0 × 10^{-15}) = 3 × 10^8 m/s, matching the speed of light.

Why the others are wrong:

• A: λ / T = 4.8 × 10^8 m/s, exceeds c.
• B: λ / T = 0.48 m/s, far below c.
• D: λ / T = 300 m/s, much less than c.

Final answer: C