**Core Concept**
The success of a root canal filling is determined by the elimination of infection and the prevention of future infection within the treated tooth. This is achieved by removing infected pulp tissue, disinfecting the canal, and sealing the canal with an inert material.

**Why the Correct Answer is Right**
The success of a root canal filling is best assessed by verifying the absence of infection through clinical and radiographic evaluation. A follow-up radiograph taken after the filling is placed can demonstrate the presence of any remaining infection or the presence of a periapical cyst or granuloma. Additionally, clinical evaluation involves checking for signs of infection, such as pain or swelling, and the patient's overall response to the treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too narrow in scope, as the success of a root canal filling is not solely dependent on the absence of pain.
**Option B:** This option is incorrect because it is too broad, as the success of a root canal filling is not solely dependent on the presence of a normal radiograph.
**Option C:** This option is incorrect because it is too vague, as the success of a root canal filling cannot be determined solely by a patient's report of satisfaction.

**Clinical Pearl / High-Yield Fact**
A follow-up radiograph taken after a root canal filling is placed is essential in verifying the success of the treatment and identifying any potential complications.

**Correct Answer:** A. Clinical evaluation and follow-up radiograph.