Question

why do scientists think the jellyfish are stuck in the cave?
they were trapped by a current
they are seeking cover
they are reproducing
they lost their navigation ability

Explanation:

Jellyfish are simple organisms without complex navigation systems. If they are in a cave and unable to get out, it is likely due to a lack of the ability to find their way out (navigation ability). Being trapped by a current would imply an external force that is not typically the main reason for being "stuck" in a biological context. Seeking cover is an active behavior that jellyfish, with their simple nervous systems, are not likely to engage in in a strategic way. Reproduction is a biological process that doesn't directly relate to being stuck in a cave.

Answer:

They lost their navigation ability