Concept:
The façade of a building refers to its exterior front surface. Identifying the building material depends on visual characteristics such as color, texture, and historical construction practices.
Explanation:
The building shown has a smooth, white, polished appearance.
Such a finish is characteristic of marble, which has been extensively used in Mughal architecture.
Sandstone is usually reddish or brown.
Granite is darker and coarser in texture.
Slate is commonly used for roofing rather than façades.
Thus, the most used building material visible on the façade is marble.
Final Answer: \(\boxed{\text{Marble}}\)