Answer: Which increases the concentration of hydroxide ion in the solution.
Explanation: Arrhenius Bases are the substances which on dissolution with water increases the concentration of the hydroxide ion in the solution.
It can also be defined as the base which gives hydroxide ion in the aqueous solution. Aqueous solution is the solution which has base present in it.
NaOH(aq) → Na+(aq) + OH−(aq)
Thus NaOH is considered as the Arrhenius base.