If a Snowflake user decides a table should be clustered, what should be used as...

 When deciding on a clustering key for a table, Snowflake recommends using the columns that are most actively used in the select filters. This is because clustering by these columns can improve the performance of queries that filter on these values, leading to more efficient scans and better overall query performance2. References: [COF-C03] SnowPro Core Certification Exam Study Guide