The correct option is eminent.

To determine the correct word to fill in the blank, we can analyze both the grammatical context and the meaning of the choices:

1. Grammatical Clue (Use of the article "an"):
The blank is preceded by the indefinite article "an". In English, "an" is used before words that begin with a vowel sound. Let's examine the starting sounds of the given options:
- imminent (starts with a vowel sound /ɪ/) - grammatically possible
- eminent (starts with a vowel sound /ɛ/) - grammatically possible
- prominent (starts with a consonant sound /p/) - incorrect (requires the article "a")
- dominant (starts with a consonant sound /d/) - incorrect (requires the article "a")

2. Vocabulary and Meaning:
Next, we evaluate the definitions of the two grammatically viable options to see which fits the context of describing a writer:
- Eminent: This adjective is used to describe a person who is famous, highly respected, and distinguished within a particular field or profession. A highly regarded author is appropriately described as "an eminent writer."
- Imminent: This adjective is used to describe an event (often something unwelcome) that is about to happen or take place very soon (e.g., "imminent danger" or "imminent arrival"). It cannot be used to describe a person's professional status.

Therefore, eminent is the only word that is both grammatically correct and contextually appropriate to complete the sentence: "John Thomas, an eminent writer, passed away in 2018."