It is easy to observe that
B-tree has secondary index
Natural join is Relational Algebraic Operation
Closure of set of attributes is related to Functional Dependency
Non-procedural Query language is related to Domain Calculus
Hence ans is D
Primary index:
A primary index is an index on a set of fields that includes the unique primary key for the field and is guaranteed not to contain duplicates. eg. Employee ID can be Example of it.
Secondary index:
A Secondary index is an index that is not a primary index and may have duplicates. eg. Employee name can be example of it. Because Employee name can have similar values.