Final answer:
An inner join is the type of join that only returns matched records from the tables being joined, excluding unmatched records which would typically be included in an outer join.
Therefore, the correct answer is: option b. inner join
Step-by-step explanation:
Inner joins combine records from two tables whenever there are matching values in a field common to both tables.
This type of join compares each row of the first table with each row of the second table to find all pairs of rows which satisfy the join predicate. If the join predicate is satisfied by matching non-null values, a row will be included in the result set.
Unlike an outer join, an inner join does not return unmatched rows. In an outer join, rows that do not find a corresponding match in the other table are still included in the result set, with NULLs for unmatchable columns.
Both equijoin and theta join are types of joins that can be either inner or outer joins, depending on the specific conditions applied.