Answer: The correct answer is "the frequency has increased".
Step-by-step explanation:
Frequency is the number of vibrations in one second. Pitch depends on the frequency. High frequency produces high pitched note. Low frequency produces low pitched note.
In the given problem, a singer raises his pitch one octave. Here, the quality of the sound wave will change. On raising the pitch the frequency will increase.
Therefore, the correct option is " the frequency has increased".