Answer: a. acid
Step-by-step explanation:
a. Acids are those substances which either donates hydrogen ions
when dissolved in water or accepts a pair of electrons.

b. Buffer is a solution which resist changes in pH on addition of small amounts of acid or base into it.
c. Bases are those substances which either donates hydroxide ions
when dissolved in water or donates a pair of electrons.

d. Indicators are chemical compounds which show a change or appearance in color at particular pH. Example: litmus