Yes, this is a valid assertion and reason. Decomposition of vegetable matter into compost is an example of exothermic reactions because exothermic reactions are those reactions in which heat is evolved.

Explanation:

Exothermic Reactions:
Exothermic reactions are those reactions in which heat is evolved or released during the reaction. These reactions usually have a negative change in enthalpy or heat of the reaction. They are characterized by a decrease in the energy of the reactants as they convert into products.

Decomposition of Vegetable Matter:
The decomposition of vegetable matter into compost is an example of an exothermic reaction. During this process, microorganisms break down the organic matter in the presence of air and water, releasing energy in the form of heat. This heat is an indication of the exothermic nature of the reaction. The compost produced is rich in nutrients and is used as a natural fertilizer.

Factors that Affect Exothermic Reactions:
Several factors can affect exothermic reactions, such as:

1. Temperature: Increasing the temperature of the reactants can increase the rate of reaction and the amount of heat released.
2. Concentration: Increasing the concentration of reactants can increase the rate of reaction and the amount of heat released.
3. Catalysts: The use of catalysts can increase the rate of reaction without affecting the amount of heat released.
4. Surface Area: Increasing the surface area of the reactants can increase the rate of reaction and the amount of heat released.

Conclusion:
In conclusion, the decomposition of vegetable matter into compost is an example of an exothermic reaction. Exothermic reactions are those reactions in which heat is evolved or released during the reaction. The factors that affect the rate of exothermic reactions include temperature, concentration, catalysts, and surface area.

Both A and R are true and R is correct reason of A