In music, the term "octave" refers to the interval between two notes where the frequency of one note is double the frequency of the other. So, in the sentence, "A note is the octave of another note if it has," the correct option is "a frequency twice that of the first note." This means that if one note has a certain frequency, the note that is an octave higher will have a frequency that is exactly double that of the first note. For example, if one note has a frequency of 440 Hz, the note that is an octave higher will have a frequency of 880 Hz.