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First, find the wavelength: c = fλ => λ = c/f = (3 × 10^8) / (750 × 10^12) = 4 × 10^-7 m = 400 nm. The fringe spacing (β) is given by β = λL/d, where L is the screen distance (4.5 m), and d is the slit separation. β = 15 mm = 15 × 10^-3 m. Therefore, d = λL/β = (4 × 10^-7 × 4.5) / (15 × 10^-3) = 1.8 × 10^-6 / 15 × 10^-3 = 1.2 × 10^-4 m = 0.12 mm. None of the options match this. Let's check the frequency: 750 THz = 750 × 10^12 Hz. λ = 3e8 / 750e12 = 4e-7 m. β = 15 mm = 0.015 m. d = λL/β = (4e-7 * 4.5)/0.015 = 1.8e-6 / 0.015 = 1.2e-4 m = 0.12 mm. Still no match. There might be a mistake. If the separation of maxima is 15 cm, then d = 1.2 mm. If the frequency is 750 Hz, then λ = c/f = 3e8/750 = 400,000 m. This is not possible. The frequency is likely 750 THz. The separation of maxima is probably 15 mm. There's a discrepancy. If d = 1.00 mm, then λ = βd/L = 0.015 * 0.001 / 4.5 = 3.33e-6 m = 3333 nm. Not correct. If d = 0.95 mm, λ = 3167 nm. If d = 1.01 mm, λ = 3367 nm. If d = 1.10 mm, λ = 3667 nm. None of these are 400 nm. So either the frequency or the maxima separation is different.