Final answer:
The equation representing a decomposition reaction is 6NaNO3 -> 6NaNO2 + 3O2, where sodium nitrate breaks down into sodium nitrite and oxygen gas.
Step-by-step explanation:
The equation that represents a decomposition reaction is option (c) 4P + 5O2 → 2P2O5. In a decomposition reaction, a compound breaks down into its constituent elements or simpler compounds. In this reaction, phosphorus (P) combines with oxygen (O2) to form diphosphorus pentoxide (P2O5).
On the other hand, options (a), (b), and (d) represent a synthesis reaction, where two or more substances combine to form a complex product.