‘The lecturer’s mastery over his subject was seen through his fluent discourse.’ Among the given pairs of words, the pair in option (2) is the most suitable. You will notice that, though the nouns ‘oratory’ – option (1), ‘grip’ – option (3), ‘skill’ – option (4) and ‘expertise’ – option (5) are all suitable for the first blank in the sentence, the second word in each of these pairs is not suitable to fill the second blank. The second word ‘long’ in option (4) is unsuitable because a long discourse is not necessarily interesting. Note that the first word in all the five options has a positive connotation; all these words refer to the lecturer’s knowledge of his subject; but among the second words, only ‘fluent’ has an equally positive meaning. ‘Fluent’ means effortlessly smooth and rapid.