The correct answer and the effect of the author using shorter sentences in that description might be to D. communicate Jem's anxiety.
Further evidence can be extracted from the use of the word "shuddered". This word already implies that Jem shivering or tremblin probably out of fear or anxiety. After reading the rest of the passage with that information in mind, we can reach the conclussion that this passage and the use of short sentences is meant to give indirect characterization of Jem's character as an anxious character. The author gives us this information from the context, actions and speech rather than making direct statements about it.