Assertion (A): The concept of strain energy can be used to analyze a statically indeterminate structure.
Reason (R): There is a direct relationship between strain energy of a structure and the slopes and deflection caused in it.

The correct option is A Both A and R are true and R is the correct explanation of A
For analysis of indeterminate structures, the method of least work, (a special form of Castigliano's Second Theorem) is useful. If $R_i$ is the reaction component, the corresponding displacement A, is zero for a rigid support, then

${\Delta }_i=\frac{\partial U}{\partial R_i}=0$

This holds for each of the reaction components (force or moment) in the structure. This method can be used to solve indeterminate beams, trusses, arches and frames. Thus it is based on the relation of displacement (slope or deflection) with the strain energy.