Explanation:
On planet B where the magnitude of the free-fall acceleration is 1.91g , the object weighs 25.74 N.
We have
Weight = Mass x Acceleration due to gravity
On planet B
25.74 = Mass x 1.91 g
25.74 = Mass x 1.91 x 9.81
Mass = 1.37 kg
Mass is constant for an object. It will not change with location.
Mass of object in Earth = Mass of object in Planet B
Mass of object in Earth = 1.37 kg