Balance is a principle of design that refers to the distribution of visual weight of objects, colors, texture, and space in a composition.
Types of balance include:
Symmetrical Balance (Formal Balance): Elements are arranged equally on either side of a central axis (bilateral symmetry) or around a central point.
Asymmetrical Balance (Informal Balance): Elements are not identical on either side of an axis, but their visual weights are balanced to create a sense of equilibrium.
Radial Balance: Elements are arranged around a central point, radiating outwards or inwards from it, like spokes of a wheel or petals of a flower. This creates a strong focal point at the center.
Let's evaluate the examples:
(a) Stupa at Sanchi: The Great Stupa at Sanchi is a hemispherical dome (\textit{anda}) on a circular or square base, often surrounded by a circumambulatory path (\textit{pradakshina patha}) and railings (\textit{vedika}) with gateways (\textit{toranas}) at cardinal points. The overall form is highly symmetrical around its central vertical axis. If viewed from above (plan view), the circular elements (dome, path, railings) radiate from a central point. The arrangement of toranas at cardinal points also contributes to a sense of radial organization around the central stupa. Therefore, a stupa exhibits strong characteristics of radial balance, especially in its plan.
(b) Sydney Opera House: Designed by Jørn Utzon, it features iconic sail-like concrete shells. While it has a complex and striking form, its overall composition is more sculptural and asymmetrical than strictly radial, though it has symmetries and focal points.
(c) Humayun's Tomb, Delhi: A Mughal tomb garden complex. The main tomb structure is highly symmetrical (bilaterally and often four-fold symmetry in plan, typical of Mughal architecture) and often set within a Charbagh garden (four-quadrant symmetrical garden). While it has a central focus and symmetries, "radial balance" in the sense of elements radiating from a center like spokes is not its defining characteristic as much as the axial symmetries of the Charbagh.
(d) Great Pyramid of Giza: A massive square-based pyramid. It exhibits strong bilateral symmetry along axes passing through its apex and the midpoint of its sides, and four-fold rotational symmetry around its central vertical axis. Its form is based on a square plan, not primarily radial.
Comparing the options, the Stupa at Sanchi, with its circular plan, central focus, and elements arranged around this center (like the circumambulatory path and toranas), is the best example of radial balance among the choices. Many circular or centrally planned structures (like domes, round temples, mandalas) exhibit radial balance.
\[ \boxed{\text{Stupa at Sanchi}} \]