if my definition is correct octaves are basically playing 2 same notes at the same time, just that one note is at a higher octave than the other.

so to simplify things, its basically a power chord minus the middle note played by your ring finger.

the octave of D would be:

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|----7- <---this is also D, but at a higher octave

|----x-

|----5- <-- this note is D

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