Concept:The word inflexible means not willing to change or compromise once a decision is made.Explanation:In the sentence, "My father remains inflexible" suggests he stays firm and does not alter his stand.Among the given options, unyielding means refusing to give in or change, which matches this meaning.Certain means sure; relaxed means calm and flexible; permanent means lasting forever. None fit the context as closely.Answer:D. unyielding