In order to define the relationship between two entity sets, which of the following operations can be used?

Correct option is C

In the Entity-Relationship (ER) model, a relationship between two entity sets is established by associating entities from one set with entities from another set. The Cartesian product of two entity sets provides all possible combinations of their entities, from which the relationship can be defined.
If:
​$$E_1=\{e_1,e_2\}$$​​
and
​$$E_2=\{f_1,f_2\}$$​​
then:
​$$E_1\times E_2= \{(e_1,f_1),(e_1,f_2),(e_2,f_1),(e_2,f_2)\}$$​​
A relationship set can be viewed as a subset of the Cartesian product:
​$$\boxed{R\subseteq E_1\times E_2}$$​​
Information Booster
1. An entity set is a collection of similar entities.
2. A relationship set represents an association among entity sets.
3. For two entity sets $$E_1$$​ and $$E_2:$$​​
​$$R\subseteq E_1\times E_2$$​​
4. The Cartesian product generates all possible pairs of entities.
5. The actual relationship generally contains only those pairs that satisfy the relationship condition.
Additional Knowledge
Why the Other Options Are Incorrect?
· (a) Concatenation — Used primarily for combining strings/sequences, not for defining ER relationships.
· (b) Adding — Arithmetic operation; not an ER-model operation.
· (d) Union — Combines elements belonging to two sets; it does not establish pairwise associations.
For example, consider:
STUDENT × COURSE
This produces possible student-course pairs. A relationship such as ENROLLS can then select the pairs representing actual enrollments.
Thus:
Relationship Set ⊆ Cartesian Product of Entity Sets