Young's double-slit interference minima positions Steps:

• Wavelength λ = c / f = (3.0 × 10^8 m/s) / (1.5 × 10^{14} Hz) = 2.0 × 10^{-6} m (consistent with pattern scale).
• Fringe spacing β = λ L / d = (2.0 × 10^{-6})(4.0) / (0.8 × 10^{-3}) = 0.010 m = 10 mm.
• Minima positions from center: y_m = (2m - 1) β / 2, where m = 1, 2, 3 for first, second, third minima.
• For third minimum (m = 3): y = (5) (10 mm) / 2 = 25 mm.

Why C is correct:

• The formula y = (2m - 1) λ L / (2 d) for odd-order destructive interference gives exactly 25 mm for m = 3.

Why the others are wrong:

• A: Matches first minimum at 5 mm, not third.
• B: Matches fringe spacing β = 10 mm, not third minimum position.
• D: Matches fifth minimum at 50 mm, beyond third.

Final answer: C