Short Answer
Orbital frequency (twice the orbital frequency in the dominant quadrupole mode) is the best answer.
Astrophysics questions often combine observation with first-principles physics. The winning move is to connect the measurement being made, such as luminosity, spectrum, redshift, or orbit, to the physical model behind it.
Gravitational waves are emitted primarily at twice the orbital frequency f_GW = 2f_orb for circular binaries in the quadrupole approximation.
Why This Answer Is Correct
This is a Hard-level question in Astrophysics & Cosmology. The prompt is really testing whether you can connect the concept to its defining physical relationship instead of picking a nearby-but-wrong term.
Good astrophysics reasoning always asks what the telescope actually measured and what physical quantity that measurement traces.
Choices At A Glance
- A. Only mass
- B. Orbital frequency (twice the orbital frequency in the dominant quadrupole mode)
- C. Magnetic field
- D. Distance to the observer
When similar options appear on an exam, eliminate the ones that break the core law, use the wrong units, or confuse a definition with a consequence.
Topic Snapshot
Topic: Astrophysics & Cosmology
Difficulty: Hard
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.