Question

why do scientists think the jellyfish are stuck in the cave?

they were trapped by a current

they are reproducing

they lost their navigation ability

they are seeking cover

Explanation:

Jellyfish are simple organisms. If they are stuck in a cave, it is likely due to a physical force like a current trapping them. Reproduction would not cause them to be stuck. Losing navigation ability (they don't have complex navigation systems like some animals) and seeking cover (they don't have the cognitive ability to seek cover in that sense) are less likely.

Answer:

They were trapped by a current