Correct Answer: Ophthalmologist

Solution : The correct answer is the first option.

The most appropriate 'one word' for the expression "one who cures eye disease" is ophthalmologist. This term is derived from the Greek words "ophthalmos" (meaning "eye") and "logos" (meaning "study" or "science"). An ophthalmologist is a medical doctor who specialises in the diagnosis and treatment of eye diseases and disorders.

The meanings of the other options are as follows:

Prologist: This term doesn't have a specific meaning related to eye diseases.

Phonologist: A phonologist is someone who studies the sound systems of languages, particularly in linguistics. This term is not related to curing eye diseases.

Biologist: A biologist is a scientist who studies living organisms and their interactions. This term is not specific to eye diseases.

Therefore, the correct word is opthalmologist.