A substance that readily burns
when it comes into contact with a flame has the chemical property of
reactivity. The answer is letter C. Reactivity is a chemical property of a
substance. According to EPA regulations, it is normally unstable and readily undergoes
violent change without detonating. it can explode or violently react when
exposed to water, when heated, or under STP.
Answer: C. Z is an intermediate.
Explanation: The given reaction is 
And the rate equation is rate = k [X_2][Y]
From this, we can imply that either the concentration of Z is very small in comparison tot he other reactants or the reactant Z is an intermediate which is available only for the given small amount of time.
There can be various steps in the mechanism for the proposed reaction and Z can also react in any of the steps. It is not necessary that it should react in a step other than the rate determining step.
This might be possible that the activation energy for Z to react is very low But since it has made a new kind of product that is XZ, then the former cannot be true.
Pure water or an extremely weak base or acid has a pH close to 7.