Suppose an astronaut were to visit a planet where the force of gravity is half that of earth. his mass on that planet would be:

a. zero
b. the same as his earth mass
c. twice his earth mass
d. half his earth mass

"Mass" is what YOU ARE.  It doesn't matter whether you're on Earth, on the moon, on an asteroid, on a meteoroid, on the sun, swimming in chicken soup on Saturn, or in a space capsule falling from one solar system body to another one.  Your mass DOESN'T change.

Whatever mass the astronaut had after he ate that steak dinner just before he was launched, THAT's the mass he has while he's on the way to another planet, and THAT's the same mass he'll have when he gets there.  It makes no difference what the gravity is like on the other planet.  His mass will not change. (choice-B)

Only his WEIGHT changes when he's in different places.