Final answer:
To structurally connect tables by relationship, a foreign key is used. It references the primary key of another table, forming a link that facilitates operations like joins.
Step-by-step explanation:
To connect one table to another in related fields, you would use a join. In relational database systems, a join operation is used to combine rows from two or more tables based on a related column between them. However, the question poses a choice, and in the context of structurally connecting tables by establishing a relationship between them, you would use a foreign key. A foreign key is a field (or collection of fields) in one table that uniquely identifies a row of another table. The foreign key is defined in a second table, but it refers to the primary key or a unique key in the first table. This creates a link between the two tables, which allows you to perform operations like joins more effectively.