To understand the given problem, let's consider the concept of a diffraction grating and its diffraction pattern.

1. Diffraction Grating:
A diffraction grating is an optical device that consists of a large number of equally spaced parallel slits or rulings. The grating constant (d) is defined as the distance between the centers of adjacent slits.

2. Diffraction Pattern:
When a monochromatic light wave passes through a diffraction grating, interference occurs between the light waves diffracted by each slit. This interference produces a pattern of bright and dark regions called a diffraction pattern.

3. Missing Orders:
In a diffraction pattern, the bright regions are called maxima, and the dark regions are called minima. The position of the maxima is given by the equation: mλ = d sin(θ), where m is the order of the maxima, λ is the wavelength of light, d is the grating constant, and θ is the angle of diffraction.

In the given problem, it is stated that the fourth order is missing from the diffraction pattern. This means that there is no bright region corresponding to the fourth order maxima.

4. Analyzing the Missing Order:
To determine the value of b/a, we need to analyze the missing order in the diffraction pattern.

- The position of the mth order maxima is given by: mλ = d sin(θ).
- For the fourth order maxima (m = 4), we have: 4λ = d sin(θ).

Now, let's consider the situation where the fourth order is missing. In this case, the angle of diffraction (θ) will be such that sin(θ) = 0. This means that the right-hand side of the equation becomes zero.

- 4λ = d * 0
- 4λ = 0

Since λ cannot be zero, this equation is not possible. Therefore, the missing order scenario is not possible.

5. Conclusion:
Based on the analysis above, we can conclude that the missing order scenario is not possible in a diffraction grating. The value of b/a cannot be determined from the given information.

Hence, the correct answer cannot be determined, and the statement given in the question is incorrect.