Areaction occurs when one compound reacts and is broken down into different elements or simpler compounds?
combustion
decomposition
single replacement
synthesis

decomposition

Explanation:

1 . Combustion is a type of chemical reaction in which a hydrocarbon reacts with oxygen to form carbon dioxide ans water along with liberation of large amount of energy.

2. Decomposition is a type of chemical reaction in which a single reactant gives two or more than two products.

3. Single replacement is a type of chemical reaction in which a more reactive element displaces the less reactive element from its slat solution.

4. Synthesis is a type of chemical reaction in which two or more than two reactants combine together to give a single product.

d) a person who learns about the world by observing it.

explanation:

scientist are people who discover why and how things are done and made in the world.