Answer is: B. Hydroxide ions.
An Arrhenius base is a substance that dissociates in water to form hydroxide ions (OH⁻).
For example sodium hydroxide: NaOH(aq) → Na⁺(aq) + OH⁻(aq).
Another example, balanced chemical reaction: Ba(OH)₂(aq) → Ba²⁺(aq) + 2OH⁻(aq).
According to the Arrhenius definition barium hydroxide is base.
Acids and bases when react (neutralisation) produce salt and water.
Dilution formula:
mv = MV
where one side is concentration × volume BEFORE dilution and the other side is concentration × volume AFTER dilution.
(100mL) × (12 M) = (500mL) × (X)
(1200 M·mL) = (500mL) × (X)
(1200 M·mL) / (500mL) = X
2.4 M = X
They can be stopped by a piece of paper.