Final answer:
The name 'Brianna St. John' would be indexed under the letter S in alphabetical order. The alphabetic order of the given three names is abc.
Step-by-step explanation:
The first indexing unit of the name Brianna St. John is St. This means that when arranging names in alphabetical order, the name would be considered under the letter S in the alphabetical order.
The alphabetic order of the three names would be:
- The Samson Equipment Co.
 - Same Day Cleaners
 - Sam of East Ridgeway
 
Therefore, the correct alphabetic order is abc.
Learn more about Alphabetical Order