Approximate thickness of oil film to observe interference of light (due to which it looks coloured) is
(1) 10 mm        (2) 10^–3 mm             (3) 10 pm           (4) 1 cm

Dear Student

For the asked condition, the thickness of film must be of the order of the wavelength of light. 

Only if the film thickness is of the order of the wavelength of light 6000 $\stackrel{}{\mathrm{A⁰}}=6000\times {}^{}$10^-10 m =6×10^-7 m will interference be observed.
The best choice is given as,
$$10000 A⁰ =104×10^-10 m = 10^-6 m  or 10^-6×10³ mm = 10^-3​ mm

Option 2 is matched.