What issue can arise if not all motors in a cluster ignite at launch?

If not all motors in a cluster ignite at launch, one significant issue is that the rocket may veer off course. When a rocket is designed to utilize multiple motors, each motor contributes to both the thrust and stability of the flight. If one or more motors fail to ignite, the thrust produced becomes uneven, leading to an imbalance that can cause the rocket to yaw or roll. This imbalance results in a trajectory that deviates from the intended path.

In contrast, losing altitude immediately typically results from other factors, such as aerodynamic drag or lack of thrust but isn't directly related to the failure of motors to ignite. Not achieving the required velocity is also a potential consequence of missing motor ignition, but it generally speaks to overall performance rather than immediate flight dynamics. Lastly, instability during ascent may arise as a secondary consequence of uneven thrust, but the initial and most immediate effect is the veering off course due to the asymmetrical thrust at launch. Thus, the direct correlation between incomplete ignition and altered trajectory makes the first option the most accurate answer.