Okay, without giving the answer, let me suggest how 1,2,4-trimethylbenzene might rearrange to mesitylene with an acid catalyst. Although I could have written this as a simple equilibrium, as obviously the starting material on the left is simply a different resonance form than that on the right, I wanted to show how one of the resonance structures might rearrange. I don't know this is exactly what happens in the bromination reaction, isomerization of xylenes to mesitylene and toluene, rearrangement of tri-t-butylbenzene, and others, but I believe it is similar to other rearrangement reactions. Deuterobenzene is produced by protonation of benzene with D2SO4, so while this may look unusual, it is not.
Perhaps someone will be able to draw the product of the bromination reaction.