Symmetric structures are those that exhibit symmetry in their internal loading and deflections. This means that the structure looks the same on both sides of a central axis. The correct answer to the question is option 'D', which states that a structure can be classified as symmetric when it develops symmetric internal loading and deflections. Let's explore this answer in detail:

Symmetry in structures:
Symmetry is an important concept in structural engineering as it helps simplify the analysis and design process. A structure is said to be symmetric when it possesses a balance or similarity of its components or properties across a central axis or plane. Symmetry can occur in various forms, such as axial symmetry, planar symmetry, or rotational symmetry.

Internal loading and deflections:
Internal loading refers to the distribution of forces and moments within a structure. Deflections, on the other hand, represent the deformation or displacement of a structure under external loads. In a symmetric structure, both the internal loading and deflections should exhibit symmetry.

Explanation of the answer:
Option 'D' states that a structure can be classified as symmetric when it develops symmetric internal loading and deflections. This means that for a structure to be considered symmetric, the internal forces and moments acting within the structure must be distributed symmetrically. Additionally, the deflections or deformations experienced by the structure under external loads should also be symmetric.

Why the other options are incorrect:
- Option 'A' states that a structure is symmetric when its structure is symmetric. While symmetry in the structure itself is a characteristic of symmetric structures, it does not guarantee symmetric internal loading and deflections.
- Option 'B' states that a structure is symmetric when its loading is symmetric. Although symmetric loading can contribute to the overall symmetry of a structure, it does not guarantee symmetric internal loading and deflections.
- Option 'C' states that a structure is symmetric when its supports are symmetric. While symmetric supports can help maintain the symmetry of a structure, they do not determine the symmetry of internal loading and deflections.

In conclusion, a structure can be classified as symmetric when it develops symmetric internal loading and deflections. This ensures that the forces, moments, and deformations within the structure are balanced and exhibit symmetry across a central axis or plane.