**Core Concept:**
Measurement of working length in endodontics is crucial for successful root canal treatment (RCT). In immature teeth with open apices, the working length should be determined accurately to ensure proper obturation and prevent complications like root resorption or failure of root development.

**Why the Correct Answer is Right:**
In the given scenario, the correct method to measure working length in a patient with trauma to an immature tooth with an open apex is using the electronic apex locator (Option C). Electronic apex locators are highly accurate and reliable devices that can provide precise measurements, ensuring proper obturation and preventing complications.

**Why Each Wrong Option is Incorrect:**
A) Using radiographs (Option A) may not provide accurate measurements due to variations in root morphology and can lead to improper obturation.
B) Using the size of the instrument (Option B) is unreliable because the root canal size does not correlate with the apical foramen size during development.
D) Using percussion (Option D) is not a reliable method as it depends on the operator's skill and is not consistent in measuring the working length in immature teeth.

**Why the Correct Answer is Right:**
Electronic apex locators work based on resistance to an electric current between the instrument and the electrode of the device. As the instrument is advanced within the root canal, the resistance changes, and the device provides a corresponding numerical value representing the working length. This method ensures accurate and consistent measurements, ensuring proper root canal treatment outcomes.

**Clinical Pearl:**
In cases of immature teeth with open apices, it is crucial to use electronic apex locators for precise working length determination during root canal treatment. This ensures successful treatment outcomes and prevents complications like root resorption or failure of root development.