Answer:
d = 1.51 x 10⁻⁵ m = 15.1 μm
Step-by-step explanation:
We will use young's Double Slit formula here:

where,
d = width of strand = ?
λ = wavelength = 631.8 nm = 6.318 x 10⁻⁷ m
L = Screen to hair distance = 1.2 m
Y = fringe spacing = 5.02 cm = 0.0502 m
Therefore,

d = 1.51 x 10⁻⁵ m = 15.1 μm