Answer: Option (a) is the correct answer.
Explanation:
A decomposition reaction is defined as a reaction where a compound dissociates into two or more atoms.
For example, 
Whereas a chemical reaction in which two reactants combine together to result in the formation of a compound is known as a synthesis reaction.
For example, 
is a synthesis reaction.
Thus, we can conclude that the statement chemical reaction A, because the reactant is a compound identifies the decomposition reaction and describes a substance is involved.
Answer:
B. Cl.
Explanation:
These are Group 7 elements whose reactivity reduces as you go down the group. Chlorine has a smaller atoms than the rest and has a stronger tendency to grab an electron in its valency shell. This is because its nucleus is closer to the outer electrons than the other elements.
Here try this presentation, it should help :)
https://prezi.com/6ofdo3fadl9a/exothermic-and-endothermic-reactions-in-everyday-life/
The most loosely held electrons in an atom are called valence electrons.
"released from" the atoms
