Short Answer
Nano-hertz frequencies from supermassive black hole binaries 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.
PTAs like NANOGrav detect nHz gravitational wave background — expected from supermassive black hole binary inspirals across the universe.
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. Frequencies ~100 Hz (LIGO band)
- B. Nano-hertz frequencies from supermassive black hole binaries
- C. Frequencies in the MHz range
- D. CMB photon frequencies
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.