This exercise utilizes the open 6th string as well as octave chords.
In bars 10-13, we'll be playing the 6th string ringing out, but then be sure to palm mute that string starting on bar 14.
While you play the octave chords, make sure your fret-hand index finger mutes all of the unused strings. Since your pick-hand is doing a lot of strumming, it might be easy to accidentally hit unwanted strings, so muting the unused string with your index finger is absolutely critical. For example, since your octave chords are on the A and G strings, your index finger should be simultaneously resting on the Low E, D, B, and High E strings to make sure they are muted.