a better choice for related constants

a choice of values (in this case, it can be either Y if the book has been sold, or N if still in possession)

a distinct type with named constant members

a mySQL data type and not a feature of INDEXU

a named constant whose underlying type is any integral type except Char

an integral type that can only take a few predefined values

a structure with a set of static constants

a type that should represent a single thing

a type, which in this case can have one of two values, YES and NO

One of several mechanisms in C++ for creating a user-defined type. In this case, a simple integer type with one or more named values.

A Java keyword used to declare an enumerated type.

A special form of value type that inherits from System.Enum and supplies alternate names for the values of an underlying primitive type. An enumeration type has a name, an underlying type, and a set of fields. The underlying type must be one of the built-in signed or unsigned integer types (such as Byte, Int32, or UInt64). The fields are static literal fields, each of which represents a constant. The language you are using assigns a specific value of the underlying type to each field.