**Core Concept**
The question is testing the understanding of radiotherapy techniques used to manage motion artifacts in tumors, specifically those caused by breathing. Tumor motion due to respiration can lead to inaccurate radiation delivery and reduced treatment efficacy.

**Why the Correct Answer is Right**
Gating is a radiotherapy technique that synchronizes radiation delivery with the patient's breathing cycle. It involves monitoring the patient's breathing pattern using a device such as an electromagnetic sensor or a video camera. When the patient inhales or exhales to a predetermined threshold, the radiation beam is turned on or off. This ensures that the tumor is exposed to a consistent dose of radiation, even when it moves due to breathing. Gating is particularly useful in treating lung and liver tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** The arc technique is a type of radiation delivery method that involves rotating the radiation beam around the patient to deliver a uniform dose to the tumor. While it is used in radiotherapy, it is not specifically designed to counteract tumor motion due to breathing.

**Option B:** Modulation refers to the variation of radiation intensity or beam shape during treatment. It is used to optimize radiation delivery and minimize side effects, but it is not directly related to managing tumor motion.

**Option D:** Shunting is a term not commonly used in radiotherapy. In the context of cancer treatment, it might refer to the diversion of blood flow or other bodily fluids, but it is not relevant to the management of tumor motion during radiation therapy.

**Clinical Pearl / High-Yield Fact**
To remember the correct answer, recall that "gating" involves "turning on" or "turning off" the radiation beam in sync with the patient's breathing cycle, making it an effective technique for managing tumor motion due to respiration.

**✓ Correct Answer: C. Gating**