Carefully read the following statement:

Though he thought of himself as a/an ______ person, his boss's abusive behaviour made him talk back. However, as he engaged in a/an _______ with his boss, all he got in response was a/an _______, which only filled him with _____

Fill in the blanks meaningfully, in the above statement, from the following options:

The last part of the sentence, "made him talk back," indicates that he considered himself a calm person. Among the options, nonchalant means calm and relaxed. Thus, the first blank will be filled with "nonchalant".

Thus, the possible answers are A and E.

By the context of the passage, the last blank must have a word meaning a feeling of emptiness or dissatisfaction. Thus, the perfect choice for the blank is "ennui".

Hence, the answer is option A.