**Core Concept**
The optimal Doppler angle is a critical factor in obtaining accurate blood flow measurements using Doppler ultrasound. It is defined as the angle between the ultrasound beam and the direction of blood flow. This angle should be as close to 0 degrees as possible to minimize errors in velocity measurements.

**Why the Correct Answer is Right**
When the Doppler angle is close to 0 degrees, the velocity of blood flow is accurately measured. This is because the Doppler shift is directly proportional to the cosine of the angle between the ultrasound beam and the direction of blood flow. When the angle is 0 degrees, the cosine is 1, resulting in the most accurate measurement. In contrast, as the angle increases, the cosine decreases, leading to underestimation of blood flow velocities.

**Why Each Wrong Option is Incorrect**
* **Option A:** An angle of 90 degrees would result in a cosine of 0, leading to a significant underestimation of blood flow velocities and is therefore incorrect.
* **Option B:** An angle of 30 degrees would result in a cosine of approximately 0.866, still leading to some degree of underestimation of blood flow velocities and is therefore incorrect.
* **Option C:** An angle greater than 60 degrees would result in a cosine less than 0.5, leading to significant underestimation of blood flow velocities and is therefore incorrect.

**Clinical Pearl / High-Yield Fact**
To achieve the optimal Doppler angle, the ultrasound probe should be carefully positioned to align with the direction of blood flow. This may require adjusting the probe angle or location to minimize the angle between the ultrasound beam and the direction of blood flow.

**Correct Answer: C.**