Explanation:
Given that,
Wavelength of the light, 
(a) Slit width, 
The angle that locates the first dark fringe is given by :



(b) Slit width, 
The angle that locates the first dark fringe is given by :



Hence, this is the required solution.
Explanation:
R = V/I
Since this is an I/V graph, the greater the slope of the line the lower the R value.
Hence we find the line with the smallest slope, which is conductor M.
It can’t be b because that will increase the dissolving rate