The most obvious thing that makes A the answer is when you look down at the top of the right angle triangle, you will only see one surface (the diagonal surface). You WILL NOT see the back edge like B suggests because it is a vertical edge (see end view).
Look for the single surface, make sure its on the correct side, answer is A - next question please.
Once again, these figures are awful! In the end view, anything behind the triangle should be hidden (dashed) lines. I strongly suggest you throw that book away or something. Maybe pick up a graphical communication text book. The first chapter of those types of books usually deal with this kind of stuff.