The Law of Constant Composition, also known as the Law of Definite Proportions, states that a pure compound will always have the same elements in the same proportions by mass. This means that regardless of the source or method of preparation, a compound will always have the same ratio of elements.

In the given options, the Law of Constant Composition does not hold good for non-stoichiometric compounds.

Non-stoichiometric compounds are compounds that do not have a fixed ratio of elements. These compounds can vary in composition and do not follow the Law of Constant Composition. They are formed when there is a deviation from the expected stoichiometry or when there is a presence of excess or deficiency of one of the elements in the compound.

Explanation:
1. Endothermic compounds: Endothermic compounds are compounds that absorb heat during their formation. While the process of formation may involve energy absorption, it does not affect the elemental composition of the compound. The Law of Constant Composition still holds true for endothermic compounds.

2. Non-stoichiometric compounds: Non-stoichiometric compounds, as mentioned earlier, do not have a fixed ratio of elements. They can have varying amounts of elements, which can lead to different compositions. This deviation from the fixed ratio violates the Law of Constant Composition.

3. Stoichiometric compounds: Stoichiometric compounds have a fixed ratio of elements and follow the Law of Constant Composition. They maintain the same ratio of elements regardless of their source or method of preparation.

4. Exothermic compounds: Exothermic compounds release heat during their formation. Similar to endothermic compounds, the release of heat does not affect the elemental composition of the compound. The Law of Constant Composition is still applicable to exothermic compounds.

To summarize, the Law of Constant Composition does not hold good for non-stoichiometric compounds, as they can have varying amounts of elements and do not follow a fixed ratio. This deviation from the fixed ratio is what sets them apart from stoichiometric compounds.