Granite is primarily composed of feldspar and quartz. During the weathering process, feldspar reacts with water and carbon dioxide to undergo chemical decomposition, leading to the formation of clay minerals and eventually a residue of gibbsite, which is an aluminum hydroxide mineral.

In the provided ternary diagram:

• The vertices represent the molecular proportions of \(\text{Al}_2\text{O}_3\), \(\text{CaO} + \text{Na}_2\text{O}\), and \(\text{K}_2\text{O}\).
• The path of weathering can be traced in this diagram.

Let's analyze the options:

• The correct weathering sequence should begin with the alteration of feldspar, which would initially produce clay minerals by reacting with water and other elements. This process would be followed by the formation of a residual product, primarily richer in aluminum, such as gibbsite.
• Path c correctly represents this transition, showing the weathering progression from feldspar through clay minerals to the aluminum-rich residue gibbsite, moving towards the \(\text{Al}_2\text{O}_3\) vertex.

Therefore, the correct weathering path for the granite rock in the given diagram is option c.