Re: the question of brightness?

Although the intensity of light depends on the no. of photons and their frequency, the photon number usually contributes more to the intensity than the frequency. Notice that visible light spectrum ranges from 400 nm - 700 nm, the maximum difference in wavelengths (and hence frequencies) from violet to red is less than double. However, the no. of photons could be increased/decreased by many many times (ten to thousand times or more).

In the question, X and Y are ADJACENT spectral lines. I would expect the difference in frequencies between X and Y is not large. The intensity thus depends on the no. of photons more than on the frequencies. Option D gives the fact that more electronic transitions between two states indicates more photons emitted, hence leading to higher intensity. Option E is not correct, as the result of which gives the frequency of the spectral line, not much to its intensity. Other options are obviously incorrect.

Since the two spectral lines are close to each other (i.e. with not large frequency difference), variation of human eye sensitivity to frequency doesn't need to be accounted for.

If you are interested, you may read the following web-pages on brightness and intensity of light:

http://forums.studentdoctor.net/threads/brightness-of-light-and-frequency.991378/

http://physics.stackexchange.com/questions/16910/whats-the-difference-between-dim-and-bright-light

The diagram below gives how the sensitivity of human eyes varies with light frequency (or wavelength):

https://en.wikipedia.org/wiki/File:Eyesensitivity.svg

(Note: I think in high school physics, eye sensitivity to colours of light (or frequencies) is normally not well taken care of. You may, sometimes, regard intensity is more or less the same as brightness, though this is not exactly correct in physics).