To solve this question, the user needs to have a basic understanding of databases and the different methods used to assign unique numbers to columns in a database.

Now, let's go through each option and explain why it is right or wrong:

A. Clusters: Clusters are used to group related data together in a database. They are not used to assign unique numbers to columns in a database. Therefore, this option is incorrect.

B. Synonym: Synonyms are used to provide an alternative name for a table or view in a database. They are not used to assign unique numbers to columns in a database. Therefore, this option is incorrect.

C. Views: Views are virtual tables that are created by combining data from one or more tables in a database. They are not used to assign unique numbers to columns in a database. Therefore, this option is incorrect.

D. Sequences: Sequences are used to generate unique numbers for a column in a database. They are often used to assign primary keys to tables in a database. Therefore, this option is correct.

The Answer is: D