Which one of the following is not an agent of metamorphism?

Which one of the following is not an agent of metamorphism?

Heat
Compression
Decomposition
Solution
This question was previously asked in
UPSC NDA-2 – 2018
Metamorphism is the process by which existing rocks are transformed into new types of rock by high temperature, high pressure, and/or chemically active fluids. Heat, pressure (often resulting in compression), and chemically active fluids (which can involve solution and redeposition of minerals) are the primary agents of metamorphism. Decomposition, which is the breakdown of substances, typically organic matter, is not a process that drives the transformation of rocks under metamorphic conditions.
The three main agents of metamorphism are heat, pressure, and chemically active fluids. These agents cause mineralogical, textural, and/or chemical changes in the parent rock without complete melting.
Heat can be provided by geothermal gradient, contact with magma, or friction along faults. Pressure can be lithostatic (confining pressure from overlying rocks) or directed pressure (differential stress). Chemically active fluids are typically hot water or gas mixtures that circulate through the rock, dissolving and transporting ions, leading to the formation of new minerals. Decomposition is a biological and chemical process associated with weathering and breakdown, distinct from the high-energy transformations of metamorphism.