Answer:
![\boxed{\text{D. Unbalanced force}}](https://img.qammunity.org/2020/formulas/chemistry/middle-school/gqigw15a46rs9oana2xetn87nfcye2kg52.png)
Step-by-step explanation:
If a rocket is at rest or moving at a constant velocity, the forces on it are balanced.
If the engines are ignited in flight, the thrust from the propellant gases unbalances the forces, and the rocket speeds up.
A is wrong. There can't be zero net force if the rocket is accelerating.
B is wrong. There is no friction in space.
C is wrong. The forces can't be balanced if the rocket is accelerating.