**Core Concept**
The question is testing the understanding of the mechanical advantage of a force applied through a canine retractor, a tool used in oral surgery to retract and expose the surgical site. The canine retractor is designed to apply a force in a specific direction, which can be measured in terms of its mechanical advantage.

**Why the Correct Answer is Right**
When a 300 gm force is applied through a canine retractor, the mechanical advantage of the retractor is utilized to amplify the force. The retractor's curved shape and angled design allow it to apply a greater force in the direction of the surgical site, making it easier for the surgeon to access the area. The exact mechanical advantage of the retractor depends on its design and dimensions.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the effect of the canine retractor on the applied force.
* **Option B:** This option is incorrect because it implies a change in the direction of the force, which is not the primary effect of the canine retractor.
* **Option C:** This option is incorrect because it suggests a change in the magnitude of the force without considering the mechanical advantage of the retractor.

**Clinical Pearl / High-Yield Fact**
When using a canine retractor, surgeons should be aware of its mechanical advantage to apply the correct amount of force and avoid damaging surrounding tissues.

**Correct Answer: D.**