To expiate means to make amends or reparation for (guilt or wrongdoing). For example, "their sins must be expiated by sacrifice". Hence, Option C is correct. Among the rest of the options, apologise means express regret for something that one has done wrong, ingratiate means express regret for something that one has done wrong, propitiate means win or regain the favour of (a god, spirit, or person) by doing something that pleases them. These options do not express the intended meaning, hence incorrect.