Choose the appropriate word to fill in the blank: It was a challenge to _______ a response from the prisoner.

The correct option is elicit.

To choose the correct word for the blank, we need to understand the meanings and spellings of the given options:
1. Elicit (verb): This means to draw out, evoke, or obtain a reaction, answer, or fact from someone. In the context of the sentence, "to _______ a response from the prisoner," the action of drawing out or obtaining a response fits perfectly. Therefore, "elicit" is the correct word.
2. Illicit (adjective): This means forbidden by law, rules, or custom (illegal). Since the blank requires a verb after "to" (infinitive form "to [verb]"), an adjective like "illicit" is grammatically incorrect here, and its meaning does not fit.
3. Licit (adjective): This means lawful or legal. Similar to "illicit", it is an adjective and grammatically incorrect for this blank.
4. Ellicit: This is a common misspelling of the word "elicit" and is not a valid English word.

Thus, the appropriate sentence is: "It was a challenge to elicit a response from the prisoner."