Answer: A. a substance that is alkaline
Explanation: Base is defined as the substance which is alkaline in nature.
It is the species which can accept a proton because of the lone pair of electrons or vacant orbitals present on them.
It is that species which donates its OH- ions to the solution (Arrhenius Base)
It is a substance which has a high pH value as low pH values belongs the the acidic species.
A species which can donate its H+ ion is known as Acidic species.