In equal temperament, C2, two octaves below middle C, has a frequency of approximately 65.406 Hz when A4 is tuned to 440 Hz.
Twelve-tone equal temperament divides each octave into 12 equal logarithmic semitones. Starting from A4 at 440 Hz, the frequency of a note is calculated with a power of the twelfth root of two. C2 is MIDI note 36, while A4 is MIDI note 69, so C2 is 33 semitones below A4: 440 × 2^(−33/12), which gives about 65.406 Hz.
The distractors represent nearby octave landmarks. Middle C, C4, is about 261.626 Hz; C3 is about 130.813 Hz; and C5 is about 523.251 Hz. Each octave doubles frequency, so the sequence makes the answer easy to check: 65.406, 130.813, 261.626, and 523.251 Hz.
The value is not universal for every tuning system. It assumes twelve-tone equal temperament and the A440 reference. Other temperaments, historical pitch standards, or a different reference frequency can produce a slightly different frequency for the same written note.