D. Determine the nature and cause of misstatements.

When encountering unexpected misstatements during an audit, the first step is to understand the reason behind these misstatements to determine whether they are indicative of a broader issue or an isolated occurrence. This helps in assessing the potential impact on the overall audit opinion. Once the nature and cause of the misstatements have been identified, the auditor can then decide on further actions, which might include expanding the sample size or changing the sampling method if necessary. Therefore, option D is the most appropriate initial action, as it sets the foundation for deciding on the next steps, which could potentially include choices A or C, depending on the auditor's findings.