The correct option is D The unit of cλ is same as that of xλ
Here, (2πctλ) as well as (2πxλ) are dimensionless. So, unit of ct is same as that of λ. Unit of x is same as that of λ. Also, [2πctλ]=[2πxλ]=M0L0T0
Hence, [2πcλ]=[2πxλt].
In the option (d), xλ is unitless. This is not the case with cλ.