Stairway to Heaven lead guitar solo!
This track uses 4 chords from the key of C major.
Am /G /F /Dm
You can use the C major scale to improvise on this, but if you start the scale from the note A (the 6th note in the scale) you get a very cool sounding minor scale called A aeolian minor.
If you play this at 5th position on the guitar, you will find some very tasty sounding stuff.