Which of the following characters cannot be used as a delimiter?

, → The comma character is commonly used as a delimiter, especially in contexts like separating column names in SQL queries or values in CSV (Comma-Separated Values) files.
. → The period character is not typically used as a delimiter in SQL queries; instead, it is often used in contexts like specifying schema.table or table.column in SQL statements.
; → The semicolon character is widely used as a delimiter to terminate SQL statements in MySQL and many other SQL database systems.
\ → The backslash character is not used as a delimiter in SQL queries; it's often used for escaping special characters or indicating special sequences in strings.

Therefore, the character that cannot be used as a delimiter is Option D: .