Comprehensive and Detailed Explanation From Exact Extract:

Spectral Doppler aliasing occurs when the Doppler frequency shift exceeds the Nyquist limit. The Doppler shift is proportional to the cosine of the Doppler angle. Increasing the Doppler angle (moving closer to 90 degrees) reduces the cosine value, thus reducing the Doppler shift frequency and helping to avoid aliasing.

According to sonography instrumentation reference:

''Increasing the Doppler angle reduces the Doppler shift frequency, thus decreasing the likelihood of aliasing in spectral Doppler.''

Therefore, the correct answer is A: Increase Doppler angle.

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