They turned down the music so as not to wake the children.
They turned down the music to avoid waking the children.
c) so that not - unusable in this sentence
d) to prevent - they can't 'prevent' the children waking up unless they drugged them - something else might rouse them, or they just wake up normally.
2) You must clean the cut thoroughly so as to prevent any infection.
You must clean the cut thoroughly to prevent any infection.(colloquial short form of 'so as to')
We use this ointment forthe prevention of infection