[A]: For accurate measurement of Doppler echocardiography the frequency of ultra- sound waves should be noticed.
[R]: Ultrasound beam can be as close to parallel.

Options

A. If both A and R are true and R is the correct explanation of A
B. If both A and R are true but R is not the correct explanation of A
C. If A is true and R is false
D. If both A and R are false

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that for accurate measurement in Doppler echocardiography, the frequency of ultrasound waves should be noted. This is correct because the frequency of the ultrasound waves is crucial in determining the speed and direction of blood flow using the Doppler effect.

2. **Understanding the Reason**: The reason provided states that the ultrasound beam can be as close to parallel. This is also true as a parallel ultrasound beam can enhance the accuracy of measurements in echocardiography by reducing the effects of angle on the Doppler shift.

3. **Evaluating the Relationship**: While both the assertion and the reason are true, we need to determine if the reason correctly explains the assertion. The assertion focuses on the importance of frequency in Doppler echocardiography, while the reason discusses the characteristics of the ultrasound beam. Therefore, the reason does not directly explain why the frequency should be noted for accurate measurements.

4. **Final Conclusion**: Since both the assertion and reason are true but the reason does not explain the assertion, we conclude that the correct answer is:
- Assertion is true.
- Reason is true.
- Reason is not the correct explanation of the assertion.