If an item set ‘XYZ’ is a frequent item set, then all subsets of that frequent item set are

The correct answer is C. frequent.

A frequent item set is an item set that occurs in at least a minimum number of transactions in a database. All subsets of a frequent item set are also frequent item sets, because they must also occur in at least the minimum number of transactions.

For example, if the item set $XYZ$ is frequent, then the item sets $X$, $Y$, $Z$, $XY$, $XZ$, $YZ$, $XYZ$, and $XZY$ are also frequent.

Option A is incorrect because all subsets of a frequent item set are also frequent item sets.

Option B is incorrect because all subsets of a frequent item set are also frequent item sets.

Option D is incorrect because it is possible to say whether or not all subsets of a frequent item set are frequent.