Select the most appropriate option to substitute the underlined segment in the…

Concept: The exclamatory pattern "What a/an + noun" expresses strong feeling about a person or thing and always needs a NOUN right after the article — "a/an" cannot be followed by a bare adjective with nothing after it. A separate pattern, "How + adjective," expresses similar feeling but takes NO article at all (e.g., "How wonderful!"). The two patterns are not interchangeable piece by piece — grafting the article from one onto the structure of the other produces an ungrammatical sentence.

Application: The underlined segment "What an inspiring !" pairs the article "an" (from the "What a/an + noun" pattern) with the adjective "inspiring" and supplies no noun, so it breaks the pattern. Replacing the adjective with its noun form, "inspiration", completes the pattern correctly. Corrected sentence: At the age of 83, she published her first book. What an inspiration!

Cross-check against the near-miss options:

• Keeping the adjective and only swapping the article ("a" for "an") still leaves no noun after it — the missing-noun problem is untouched.

• Grafting the article "an" onto the article-free "How + adjective" pattern produces a hybrid that is wrong on both counts.

• Treating the original as already correct ignores that an article directly followed by a bare adjective, with nothing after it, is exactly what the "What a/an + noun" rule forbids.

• A fully article-free rewrite, "How inspiring!", is also grammatical, though it is not among the offered substitutions here.

The noun form that completes the "What a/an + noun" pattern is the option that correctly substitutes the underlined segment.