Final answer:
The sentence that uses hyphens correctly is option a) Frankie and Johnnie had been sweethearts for twenty-three years.
Step-by-step explanation:
The sentence that uses hyphens correctly is option a) Frankie and Johnnie had been sweethearts for twenty-three years.
Hyphens are used in compound numbers from twenty-one to ninety-nine, such as twenty-three in this case.
Option b) is incorrect because hyphens should not be used for the year 1999. Option c) is incorrect because hyphens are not used to join two individual names. Option d) is incorrect because hyphens should not be used to separate clauses in this way.
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