Final answer:
The correct sentence for formal writing is 'We attended a marketing and sales conference in Key West, Florida'. Ampersands are more informal and using the full name 'Florida' is more formal than the abbreviation 'FL'.
Step-by-step explanation:
In formal writing, consistency, clarity and correctness are paramount. The sentence that fits the bill correctly for formal writing is 'We attended a marketing and sales conference in Key West, Florida'. This is because the ampersand in option 'a' is more suited for informal or business contexts. As for option 'c', fully spelling out the state's name, 'Florida', instead of using the abbreviation 'FL', is more appropriate in formal writing. As an example, 'We conducted a survey of 300 students at a large state university in the southern United States' is similar in style and conveys information clearly and precisely as a formal sentence should.
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