The term "ironic" refers to expressing meaning using language that normally signifies the opposite, often for humorous or emphatic effect.
It involves a form of sarcasm or disguised mockery, making "disguisedly sarcastic" the most accurate synonym among the given choices.
Option (A), "Inflexible," relates to rigidity and unwillingness to change, which is unrelated to irony.
Option (B), "Bitter," refers to a sharp or resentful tone but does not specifically convey the disguised or opposite-meaning element of irony.
Option (C), "Good-natured," is actually opposite in tone to irony, as irony often carries a mocking undertone.
Thus, option (D) matches the definition of "ironic" best.