To solve this question, the user needs to know SQL syntax and the concept of sorting query results in a descending order based on a specific column.

Now, let's go through each option and explain why it is right or wrong:

A. SELECT * FROM Persons ORDER FirstName DESC This option is incorrect because the correct syntax for sorting query results in SQL requires the use of the "ORDER BY" clause followed by the column name to sort by, and then the keyword "DESC" to specify descending order. Therefore, the correct syntax should be "ORDER BY FirstName DESC" instead of "ORDER FirstName DESC".

B. SELECT * FROM Persons ORDER BY FirstName DESC This option is correct. The "ORDER BY" clause is used to sort the query results based on a specific column. In this case, we want to sort the results in descending order based on the "FirstName" column. Therefore, the correct syntax is "ORDER BY FirstName DESC".

C. SELECT * FROM Persons SORT BY 'FirstName' DESC This option is incorrect because there is no "SORT" keyword in SQL. The correct keyword to use is "ORDER BY". Additionally, the column name should not be enclosed in single quotes. Therefore, the correct syntax should be "ORDER BY FirstName DESC" instead of "SORT BY 'FirstName' DESC".

D. SELECT * FROM Persons SORT 'FirstName' DESC This option is incorrect because the correct keyword to use for sorting query results in SQL is "ORDER BY" rather than "SORT". Additionally, the column name should not be enclosed in single quotes. Therefore, the correct syntax should be "ORDER BY FirstName DESC" instead of "SORT 'FirstName' DESC".

The Answer is: B