In Principle, If Astronomers Were in the Vicinity of a Schwarzschild

Multiple Choice

In principle, if astronomers were in the vicinity of a Schwarzschild black hole, how could they measure its mass?

A) Put a satellite in orbit around it and measure its period.
B) Measure the gravitational redshift from its radiation.
C) Use charged particles to measure the strength of its magnetic field.
D) Measure the speed necessary to pass through the ergoregions and emerge.