a = peak value of Px = 2/l (i.e., the value of Px at those x for which sin2() = 1)
The wavelength = the length of 2 humps = l/2 for an n = 4 electron.
Probability = area relative to the whole area under the curve which is 1
The area for the space from x = l/2 to x = (3/4)l = 1/4
Wave function or j = Px1/2 is given in the figure.
Answer is C