small concentrations of acid left in a system from a prior problem can very quickly accelerate the formation of additional acid. this acid formation is further accelerate if _ is present in the system

Catalyst

This acid formation is further accelerate if CATALYST is present in the system.

Explanation:

Catalysts are compounds that are introduced into a chemical system or reaction in order to accelerate the progress of a reaction and the catalyst itself is not consumed.

Catalysts accelerate reactions by actually reducing the energy of the rate-limiting transition state and it does not affect the equilibrium state of the reaction.  

Common examples of catalysts are acid catalysts and enzymes.

the correct answer is (a) . answer should be simple, clear and brief.