Scratch

1. Use broadcasts.

when gf clicked
broadcast [scene1 v]
when I receive [scene1 v]
switch to costume [scene1 v]
when I receive [scene2 v]
switch to costume [scene2 v]

^ In stage.

when gf clicked
hide

^ In every sprite.

when I receive [scene1 v]
show
when I receive [scene2 v]
hide

^ In sprites which appear in scene 1.

when I receive [scene2 v]
show
when I receive [scene1 v]
hide

^ In sprites which appear in scene 2.

You could easily extend this to many scened-projects.

2. Create different sprites, each of which have one of the choices.
Also, have a list of the correct choices (1, 2, 3).

when gf clicked
set [question# v] to (1)
broadcast [showquestion v]

^ In the stage.

when I receive [showquestion v]
switch to costume (question#)

^ In each of the 'choice' sprites.

when [choice_1] clicked
if < (item (question#) of [answers v]) = (1)>
change [correct v] by (1)
say [Correct!] for (2) secs
else
change [incorrect v] by (1)
say [Incorrect!] for (2) secs
end
change [question# v] by (1)
broadcast [showquestion v]

^ In the 'choice' sprite 1.

when [choice_2] clicked
if < (item (question#) of [answers v]) = (2)>
change [correct v] by (1)
say [Correct!] for (2) secs
else
change [incorrect v] by (1)
say [Incorrect!] for (2) secs
end
change [question# v] by (1)
broadcast [showquestion v]

^ In the 'choice' sprite 2.

when [choice_3] clicked
if < (item (question#) of [answers v]) = (3)>
change [correct v] by (1)
say [Correct!] for (2) secs
else
change [incorrect v] by (1)
say [Incorrect!] for (2) secs
end
change [question# v] by (1)
broadcast [showquestion v]

^ In the 'choice' sprite 3.

question# is a variable which determines which question it is.
The broadcast [showquestion] will show the next question.

3. There are many projects on the Scratch website which have an enemy which contains scripts for artificial intelligence.
Could you specify how you want the enemy to move, and what it should (optimally) do?

Something like this. You'd have to tweak the scripts to fit your project though.
Set the AI's rotation to "left-right".

when gf clicked //in the character
forever
  move (10) steps
  if on edge, bounce
  if <touching [character v]?>
    broadcast [die v]

when I receive [die v] //in the Stage
switch to background [sorry, you lost v]
stop all