Final answer:
The best field length for a field whose value is the name of one of the 12 months is 9 characters.
Step-by-step explanation:
The best field length for a field whose value is the name of one of the 12 months would be 9 characters. This is because the longest month name, September, has 9 characters. By making the field length 9, you ensure that all the month names can fit without any truncation. A field length of 6 would cut off the longer month names, while a field length of 12 or 15 would be unnecessarily long.