For the orbital speed of a spacecraft we know that

here
M = mass of the planet around which satellite is revolving
r = orbital radius
Now when thruster is used by the spacecraft its speed will change due to which orbital speed will change.
Since here while changing the speed mass of the planet will be same
we can say the speed of the spacecraft will changed by thruster due to which its orbital radius will change
so the correct answer must be
b. the spacecraft will change motion and will maintain this new orbit until the thruster is fired again.