Which of the following is NOT a type of SQL constraint?

Answer: Option B

An ALTERNATE KEY is not a type of SQL constraint. The other options, PRIMARY KEY, FOREIGN KEY, and UNIQUE, are all types of constraints used in SQL:
PRIMARY KEY ensures that a column or set of columns uniquely identifies each row in a table.
FOREIGN KEY establishes a link between two tables, enforcing referential integrity between them.
UNIQUE constraint ensures that values in a column (or set of columns) are unique across the table, but it doesn't require that the column(s) be the primary means of identifying rows.
Option B, ALTERNATE KEY, is not a standard SQL constraint term.