$exists is correct because it is the MongoDB query operator that checks for the presence of a field and so it is used to retrieve every record that contains a particular key.

The $exists operator matches documents where the specified field is present when set to true and it can be combined with other criteria to narrow results based on both presence and values.

$in is incorrect because it tests whether a field's value equals any value in a given array and it does not check whether the key itself exists.

Google Firestore is incorrect because it names a different document database product and it is not a MongoDB query operator that you would use to test for a key in a MongoDB compatible interface.

$type is incorrect because it matches documents by the BSON type of a field and it does not solely test for the existence of the field.

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