C

In Young's double slit experiment, the intensity of light at a point on the screen can be calculated using the formula:
I = I_0 (1 + cos(\delta)), where I_0 is the maximum intensity and \delta is the phase difference related to the path difference \Delta x.
The phase difference \delta is given by \delta = \frac{2\pi}{\lambda} \Delta x.
Given that for the path difference \Delta x = M\lambda, the intensity is I = I_0 (1 + cos(2\pi M)).
Since cos(2\pi M) = 1 for integer M, we have I = 2I_0.
For the second path difference \Delta x = n\lambda, the intensity is I' = I_0 (1 + cos(2\pi n)).
Setting I' = nM:
nM = 2I_0:
Thus, n = 2.
Therefore, the answer is C.