The correct answer is A. Chain.
A chain database structure is a type of hierarchical database structure in which each record is linked to only one other record. This can be inefficient for large databases, as it can lead to a lot of wasted space.
A network database structure is a type of hierarchical database structure in which each record can be linked to multiple other records. This is more efficient than a chain database structure, but it can be more difficult to manage.
A tree database structure is a type of hierarchical database structure in which each record can only be linked to one parent record, but can have multiple child records. This is more efficient than a chain database structure, but it can be more difficult to manage than a network database structure.
A relational database structure is a type of database structure in which data is stored in tables. Each table has a set of columns and rows. The columns represent the attributes of the data, and the rows represent the individual records. This is the most common type of database structure, and it is the most efficient for large databases.