Final answer:
The opposite of ephemeral is permanent, which means enduring or long-lasting, fitting as the antonym for something transient.
Step-by-step explanation:
The opposite of ephemeral is permanent. Ephemeral means lasting for a very short time, transient or fleeting. Therefore, something that is the opposite would be enduring or long-lasting which is aptly described by the word permanent. Options 2) reasonable, 3) apparent, and 4) thoughtful do not relate to the concept of time duration and therefore would not be suitable antonyms for ephemeral.