Because is used to focus on the cause of the action.
ex-Because my brother was ill,I couldn't go for playing.
Note: If emphasize the reason because will be used in the last.
ex-He spoke quitely because he didn't want to hurt him.
As/Since is used to focus on the result.
ex-Since I am hungry,I want to eat first.
Note: Since is used to refer 'from that time'.
As is used to refer 'sameness'.
ex- I set up dinner as you are tired.
For is used for poetic expression.
Option B: 'were' is used to represent(for singular) that is not possible but you wish.
ex-If I were a bird.
Option C: adjective(real) is describing another adjective(useful) which is wrong.Adverb form(really) should be used.
Option D: correct form is would have been.It means you think in present that if it happend which actually didn't.
ex-If they had left earlier, they would have been here already.
(Meaning: They didn't leave earlier, so they are not here now.)