In the absence of a gravitational field, you could determine the mass of an object (of unknown composition) by: A) applying a known force and measuring it's acceleration.
B) measuring the volume.
C) weighing it.

A) By applying a known force, and measuring it's acceleration.

Explanation:

This is actually something that astronauts do in space as a mathmatical exercise when calculating the mass of an object since F = m × a.

Once the force, and acceleration are applied, the only unknown is the mass which can be solved by dividing force over acceleration. This is because inertial mass is equal to gravitational mass.

the body's situation won't change the forces are equal opposit and on the same body