Question

what is a hallmark symptom of optic neuritis?
eye pain with movement
constant diplopia
gradual onset of eye pain
bilateral visual loss

Explanation:

Optic neuritis is inflammation of the optic nerve. Eye pain with movement is a classic and hallmark symptom. Constant diplopia (double vision) is more associated with other oculomotor nerve - related issues. Gradual onset of eye pain is not specific enough as a hallmark. Bilateral visual loss is not the most characteristic as optic neuritis often affects one eye initially.

Answer:

Eye pain with movement