Which of the following is an acceptable format for dating a letter?

The correct format for dating a letter is to use the full month name, followed by the day of the month and the year. This style is clear and formal, making it ideal for professional correspondence. In this case, "October 10, 2018" employs this structure effectively, ensuring clarity and formality that are important in business writing.

Using the numeric format, such as in the first option, may lead to confusion regarding the day and month, especially in international contexts where date formats vary. The third choice reverses the typical order by placing the year first, which is generally not standard in letter writing. The fourth choice, while close to being acceptable, includes an unnecessary "th" after the day, which is typically avoided in formal writing.