If you imagine taking the top right corner of the second block and twisting it to the bottom left corner of the image, you should be able to see it. The view shown in the keyhole is from the backside if the longest edge of the longest block is on the top. Sorry if this explanation is a little confusing! It's hard to explain this stuff not in person. Let me know if you want me to try to explain it another way.
If his explanation didnt help,
Try visualizing twisting the figure so that the piece all the way to the left that is vertical rectangle and rotating it so its longer part is horizontal and on the top.
Idk if this will work but..
It would look like this after you rotate it (imagine that the horizontal lines match up with the vertical lines)
|====|
|===|
|===|
|=|
And now just flip that over
Im assuming 3 is C and as for # 5 it cant be any one those...mistake in the book? but it seems like the most probable answers is A but not really because the shape doesnt add up, in the diagram it has a square base but in the A it has it as a trapazoid base which does not add up.