Short Answer
2GM/c² 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.
rs = 2GM/c² — the radius at which escape velocity equals c; material within cannot escape.
Why This Answer Is Correct
This is a Medium-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. GM/c
- B. 2GM/c²
- C. GM/c²
- D. 4GM/c²
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: Medium
Best next move: Re-state the governing law in your own words, then solve one more example from the same topic before moving on.