Which of the following chemical equations is an example of a thermal decomposition reaction?

Introduction / Context:
Decomposition reactions are one of the fundamental types of chemical reactions introduced in school level chemistry. In a decomposition reaction, a single compound breaks down into two or more simpler substances, often under the influence of heat, light or electricity. The question checks whether the learner can identify this pattern in given balanced equations and recognise that more than one example can satisfy the definition. Understanding decomposition reactions is important because they appear in thermal analysis, metallurgy, and environmental chemistry.

Given Data / Assumptions:
- Several balanced chemical equations are provided as options.
- Each option shows a single reactant breaking into simpler products or may appear to do so.
- The question asks which equations represent decomposition reactions.
- We assume reactions occur under suitable conditions such as heating.

Concept / Approach:
The core idea is that a decomposition reaction has the form AB giving A plus B or more products. The key features are one reactant and multiple products. In thermal decomposition, heat is applied to cause the breakdown. To solve the question, we look at each equation and check whether the left side has a single compound and the right side has smaller substances. If each reaction follows this general pattern, then each is an example of decomposition and the correct answer will be the option that includes all of them together.

Step-by-Step Solution:
Step 1: Examine option A: 2HgO ----> 2Hg + O2. One compound, mercury oxide, breaks down into mercury and oxygen gas, so this is a decomposition reaction. Step 2: Examine option B: 6NaNO3 ----> 6NaNO2 + 3O2. Sodium nitrate decomposes into sodium nitrite and oxygen gas. Again, a single compound yields simpler products, so it is a decomposition reaction. Step 3: Examine option C: ZnCO3 ----> ZnO + CO2. Zinc carbonate breaks down into zinc oxide and carbon dioxide, another typical decomposition example. Step 4: Since all three individual equations are decomposition reactions, the most accurate choice is that all of the above are decomposition reactions.

Verification / Alternative check:
A quick check is to count the number of reactants in each equation. In options A, B and C there is exactly one reactant on the left side in each equation. Then count the number of products. Each equation produces at least two distinct products. This simple counting rule is a reliable short cut for recognising decomposition reactions at this level. Because all three equations pass this test, it confirms that they are all decomposition reactions driven by heating, known as thermal decomposition reactions.

Why Other Options Are Wrong:
Options A, B and C individually show correct decomposition reactions, but the question asks which of the following is a decomposition reaction and includes a combined choice. Choosing only one of A, B or C would ignore the fact that the others also fit the definition. The correct comprehensive choice is option D, which states that all of the above represent decomposition reactions, reflecting that every equation shown is of that type.

Common Pitfalls:
Students sometimes assume that only very simple breakdowns like AB giving A plus B can be called decomposition, and may overlook reactions where one reactant forms more complex looking products. Another mistake is to think that the presence of coefficients such as 2 or 6 changes the type of reaction, even though they only balance atom counts. Learners may also confuse decomposition with displacement or redox reactions. Remembering that decomposition is characterised by one reactant and multiple products helps avoid these misunderstandings.

Final Answer:
All three given equations are decomposition reactions, so the correct choice is All of the above represent decomposition reactions.