Explanation:

- To expand vocabulary, relating a known word with an “unfamiliar” one helps deduce meaning from context.

- Correct Answer: Option 3: unfamiliar

- Known words act as clues to new meanings when in context with unfamiliar words.

- Option 1: unnecessary

- Contextually does not fit because unfamiliar, not unnecessary, words aid vocabulary growth.

- Option 2: essential

- Doesn’t quite work as context focus is on new and unknown (unfamiliar) terms.

- Option 4: unclear

- Unclear words might carry ambiguity but expanding vocabulary involves new, not ambiguous words.