Which operator is used to exclude specific values from a query in MongoDB?

The operator used to exclude specific values from a query in MongoDB is $nin. This operator stands for "not in" and allows you to filter documents by specifying an array of values that should not be present in a given field. When you use $nin in a query, MongoDB will return documents where the value of the specified field does not match any of the values in the array provided.

For instance, if you are querying a collection of products and want to find all products that are not in a certain category, you could use $nin with an array containing the categories you want to exclude. This makes $nin particularly useful for filtering out unwanted results based on specified criteria.

In contrast, the other operators listed serve different purposes. $in is used to include documents with values that match any values in a specified array. $nor is a logical operator that can be used to combine multiple conditions and return documents that do not meet any of the specified conditions. $not, on the other hand, is utilized to apply negation to a query condition but does not specifically handle arrays like $nin does. Therefore, for the purpose of excluding specific values from a query, $nin is the appropriate operator to use.