Answer:
The letter A is the right answer.
Step-by-step explanation:
Danish Johannes Nicolaus Brönsted (1879-1947) and Englishman Thomas Martin Lowry (1874-1936) proposed an acid-base theory known as the Brönsted-Lowry theory, which reads as follows:
Acid is a chemical substance that necessarily donates H⁺ protons, while a Base is a chemical substance that necessarily receives H⁺ protons.
In this case, the proton is considered the hydrogen ion. This is seen in the following reaction, where hydrocyanic acid donates a proton to water, which acts as a base:
HCN + H2O → CN- + H3O⁺
This reaction is reversible, and the hydronium ion (H3O +) can donate a proton to the CN- ion. Thus, hydronium ion (H3O +) acts as acid and CN- as base.
CN- + H3O⁺→ HCN + H2O