Which character is illegal in naming an unquoted identifier in SQL?

This question is asking about the rules for naming things in SQL, like tables and columns.
SQL has specific rules for naming things. One important rule is that you can't use just any character.
Let's look at the options:
Option A: . (period) Periods are used to separate parts of a name, like database names from table names. They are not allowed as part of the actual name itself.
Option B: _ (underscore) Underscores are perfectly fine to use in names! They are commonly used to make names more readable, like "customer_name".
Option C: $ (dollar sign) Dollar signs are not allowed in unquoted identifier names. They are used for something else in SQL.
Option D: 2 (number) Numbers are allowed in names, but only if they are not the first character. So, "table2" is fine, but "2table" is not.
The answer is Option A: . (period).