**Core Concept**
A Z-plasty is a surgical technique used to improve the functional and cosmetic outcome of a scar by rotating a triangular piece of skin. The ideal angle of a Z-plasty is crucial in achieving optimal results, as it affects the lengthening and realignment of the scar.

**Why the Correct Answer is Right**
The ideal angle of a Z-plasty is typically between 30° to 60°. This range allows for sufficient lengthening of the scar while minimizing the risk of complications such as dog-ear formation or excessive tension on the wound. The 30° angle is often used for scars that require minimal lengthening, while the 60° angle is used for more extensive lengthening.

**Why Each Wrong Option is Incorrect**
**Option A:** A 90° angle is too large and may result in excessive tension on the wound, leading to complications such as dehiscence or scar contraction.
**Option B:** A 15° angle is too small and may not provide sufficient lengthening of the scar, making it less effective in improving the cosmetic outcome.
**Option D:** A 120° angle is too large and may result in an excessive lengthening of the scar, leading to an unnatural appearance.

**Clinical Pearl / High-Yield Fact**
When performing a Z-plasty, it's essential to remember that the length of the limbs of the Z-plasty should be equal to achieve optimal results. This is known as the "equal limbs" rule.

**Correct Answer:** C. 60°.