Answer:
The H⁺ concentration in solution is increased by an Acid.
Step-by-step explanation:
To understand this we will simply define each given option.
An Acid:
Is any specie which when dissolved in water increases the concentration of Hydrogen Ions (i.e. H⁺) like HCl, or donated H⁺ to base like HBr or accepts lone pair electrons like BF₃.
Salt:
Salt is formed from the neutralization reaction of Acid and Base. So, when the salt is formed it decreases both the concentration of H⁺ or Hydroxide ions if present.
Base:
It is any specie which when dissolved in solution produces hydroxyl ions like NaOH, or accepts proton or donate lone pair of electrons like :NH₃.
Water:
Water is neither an acid nor a base. It is a neutral compound which on ionization produces hydroxyl and hydrogen ions at the same time which at that time neutralize each other. Hence, we can conclude that water does not increase the concentration of H⁺ ions.