The correct answer is $\boxed{\text{B}}$.
The total RMS pulse broadening is the square root of the sum of the squares of the individual RMS pulse broadenings. In this case, the individual RMS pulse broadenings are 12ns/km and 28ns/km, so the total RMS pulse broadening is:
$$\sqrt{(12\text{ ns/km})^2 + (28\text{ ns/km})^2} = \sqrt{784\text{ ns}^2} = 28.12\text{ ns/km}$$
Option A is incorrect because it is the sum of the individual RMS pulse broadenings, not the square root of the sum of the squares. Option C is incorrect because it is the square root of the sum of the squares of the individual RMS pulse broadenings, but the individual RMS pulse broadenings are not correct. Option D is incorrect because it is the square root of the sum of the squares of the individual RMS pulse broadenings, but the individual RMS pulse broadenings are not correct.