(c) 'I haven't read both' implies he has read one of them. He therefore knows himself whether that story was interesting or not. Yet the speaker goes on, 'but I heard that they are very interesting." This implies he is having to rely on the opinion of others about both stories, when in fact he is supposed to have read one of them himself ...so (c) does not fit the meaning of the sentence.
(d) he hasn't read either of the stories, and is quoting what he has heard -'they are (both) interesting'
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