Answer: I would contend that the right answer is actually the 4) "The sound grew louder--louder--louder! And still the men chatted pleasantly..."
Explanation: In literature, juxtaposition occurs when two or more characters, ideas or places are compared or contrasted with the goal of achieving a rethorical effect and make the reading more engaging, vivid and intellectually participatory for the reader. To the best of my knowledge, the first three options do not feature any juxtapositions. Only option 4 does. Even though the sound was growing louder and louder (action 1), the men continued to chat pleasantly (action 2). The writer is here juxtaposing two dissimilar actions - loud sound and pleasant chat - in order to catch the attention of and generate interest in his/her readers, who will inevitable start anticipating or at least wondering what's going to happen next.