What happens to Mach nr on a long leg when weight decreases and all other factors remain constant. Temperature, Pressure alt (FL) and thrust is constant...
First a caveat: Whenever the phrase "all other factors remain constant" appears in a test question, BEWARE - the test writer may not have considered the possibility that all other conditions CANNOT remain constant, in which case you have to crawl inside the mind of the writer to deduce what he probably meant.
But in this case, flying in a constant atmospheric mass with decreasing weight, induced drag goes down, therefore Mn rises. (Of course profile drag then rises, limiting the amount of speed increase, but faster just the same...)
What "book" tells you that speed decreases?? In the real world, you'll probably clear to a higher altitude for economy, and might then cruise slower, but that violates the premise of the quiz.