Concept:
A Relational Database Management System (RDBMS) stores data in related tables and is the most widely used database model in GIS.
It efficiently links spatial features with their attribute (non-spatial) information.
Step 1: Understand GIS databases.
An RDBMS organizes data into tables connected through common keys.
This enables efficient storage, retrieval, and querying of geographic information.
Step 2: Identify the correct database structure.
Hence,
\[
\boxed{\text{Relational (RDBMS) is the most commonly used GIS database structure}.}
\]
Therefore, the correct option is
\[
\boxed{(D).}
\]