The correct answer is (a) ml = 2d sin q, where l= lambda
To explain I would say: The expression relating the wavelength of the radiation and the angle (q) at which it is reflected is given by
ml = 2d sin q, where l= lambda.
where d is the distance separating the grooves and is known as the grating constant and m is the order of interference.
When m = 1, the spectrum is known as first order and with m = 2, the spectrum is known as second order.