Final answer:
Trajectory means the path an object takes as it moves through space, and in the provided sentence, it indicates the path of the storm that might cause the river to overflow.
Step-by-step explanation:
The term trajectory refers to the path that a moving object follows through space over time. Based on the prefix 'tra-', root 'ject', and suffix '-ory', the meaning of trajectory in the sentence 'the trajectory of the storm had changed and the river might overflow' is very close to option B, which is the path something takes as it moves over. The word combines the concept of 'throwing across' a certain space ('tra-' indicating across, 'ject' meaning throw) and a place where something occurs (-ory), which in this context is the path through the air or space.