Step 1: Reflexivity Check
A relation is reflexive if all elements relate to themselves. Here, every triangle is congruent to itself: \( \triangle A \cong \triangle A \). Therefore, relation R is reflexive.
Step 2: Symmetry Check
A relation is symmetric if \( a \) relates to \( b \) implies \( b \) relates to \( a \). If \( \triangle A \cong \triangle B \), then \( \triangle B \cong \triangle A \). Thus, relation R is symmetric.
Step 3: Transitivity Check
A relation is transitive if \( a \) relates to \( b \) and \( b \) relates to \( c \) implies \( a \) relates to \( c \). If \( \triangle A \cong \triangle B \) and \( \triangle B \cong \triangle C \), then \( \triangle A \cong \triangle C \). Thus, relation R is transitive.
Step 4: Conclusion
Relation R is reflexive, symmetric, and transitive, confirming it is an equivalence relation. Answer is (D).