Final answer:
The electric pulp test is the best test to diagnose acute apical periodontitis.
Step-by-step explanation:
The best test used to diagnose acute apical periodontitis is the electric pulp test. This test measures the electrical activity of the nerve in the tooth's pulp. In cases of acute apical periodontitis, the nerve may be damaged, leading to decreased electrical activity.
Other tests mentioned in the options, such as percussion and cavity test, may be used to assess dental health but are not specific to diagnosing acute apical periodontitis. The thermal test assesses sensitivity to hot and cold temperatures, while the anesthetic test evaluates the effectiveness of anesthesia. None of these tests directly diagnose acute apical periodontitis.