One database field needs to be restricted to only those values that are single alphanumeric...

A field restricted to single alphanumeric symbols should use the character data type because it is designed to store exactly one character. Alphanumeric symbols include letters A through Z and digits 0 through 9, and these are naturally represented as individual characters. In Information Technology and database systems, selecting a character type allows the designer to enforce a fixed length of one, which helps maintain data integrity by preventing longer entries. An integer type is used for whole numbers and would not properly handle letters, while floating point is for decimal values and is also inappropriate for single symbols. A character string type can store multiple characters, which would allow inputs longer than one symbol unless additional constraints are applied. Using the character type is therefore the most direct and standard approach for storing one letter or digit, such as a middle initial, a grade code, or a single-character category label. Therefore, the correct data type is character.