Professor John is the leader of the … committee to review the new book.

Analyze the question: The blank needs a compound adjective that describes the noun "committee" (a committee of fifty men).

Concept: When a number and a noun combine to form a compound modifier before another noun, the noun stays singular and is joined to the number with a hyphen. It never takes a plural "-s" or a possessive "'s".

Examples:

• a ten-year-old boy (not "ten-years-old")
• a five-star hotel
• a two-hour lecture

Solution:

• A. fifty-men: The noun "men" is plural, which is wrong in a compound modifier.
• B. fifty-men's: This is plural and possessive, which is wrong.
• C. fifty-man's: This is possessive, but the committee does not "belong to" a man, so it is wrong.
• D. fifty-man: The noun is singular and hyphenated, which is correct.

The sentence reads: Professor John is the leader of the fifty-man committee to review the new book.

Tip: Think of the pattern "number + singular noun + noun", as in a five-day trip or a three-man panel. If the number and noun together describe another noun, drop the "s".