According to the institute of Electrical and Electronic Engineers (IEEE) standard 519.

The Institute of Electrical and Electronics Engineers (IEEE) Standard 519, titled "Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems," sets forth guidelines to maintain the quality of electrical power by limiting harmonic distortions. According to IEEE 519, for general low-voltage systems (defined as systems with a voltage less than 600 volts), the Total Harmonic Distortion (THD) of the voltage waveform should not exceed 5%. This threshold ensures the reliable operation of electrical equipment and minimizes potential issues such as overheating or malfunctioning caused by excessive harmonics. Therefore, the correct answer is C. Less than 5%.