The question asks us to identify which among the given options is not an agent of metamorphism. Metamorphism is the process of changing the structure or composition of rocks through the application of certain agents. Let`s examine each option to understand which one is not an agent of metamorphism.

Option 1: Heat. Heat is a primary agent of metamorphism. It causes rocks to undergo changes in mineral composition and crystal structure.

Option 2: Compression. Compression refers to the application of pressure to rocks, causing them to compact and undergo changes in texture and structure. Compression is also a common agent of metamorphism.

Option 3: Decomposition. Decomposition is the process of breaking down organic matter into simpler compounds. It is not an agent of metamorphism. Alert - correct answer should be option 3, as decomposition is not related to the process of metamorphism.

Option 4: Solution. Solution refers to the dissolution of minerals in water, leading to the formation of new minerals. This process can occur during metamorphism, making solution an agent of metamorphism.

Based on the explanations above, it is clear that option 3, decomposition, is not an agent of metamorphism.