1. The problem asks which items can be analyzed. 2. To analyze means to examine methodically and in detail. 3. Rock samples and blood can be analyzed scientifically. 4. The causes of the Vietnam War can be analyzed historically. 5. The length of the day can be analyzed by measurement. 6. Therefore, all options (a), (b), (c), and (d) can be analyzed. Final answer: (a), (b), (c), (d)