Step 1: Understanding the Concept:
In any chemical reaction, the substances that undergo change are called reactants. They interact with each other, rearrange atoms, and form new substances called products.
Step 2: Analysis of Options:
Reducing agents: Specific type of reactants that reduce other substances by donating electrons.
Oxidising agents: Specific type of reactants that oxidize other substances by accepting electrons.
Reactants: General term for all starting materials of a reaction.
Products: The substances formed after the chemical reaction.
Step 3: Detailed Example:
In the reaction:
\[
2H_2 + O_2 \rightarrow 2H_2O
\]
Hydrogen (H$_2$) and oxygen (O$_2$) are reactants, while water (H$_2$O) is the product.
Step 4: Final Answer:
The substances that take part in a reaction are called Reactants.