The correct answer is option 1 - a rectifier is an electronic device used to convert AC voltage into DC voltage.
Option 2 - DC voltage into AC voltage is incorrect because a rectifier is designed to convert alternating current (AC) into direct current (DC), not the other way around.
Option 3 - sinusoidal pulse into square pulse is also incorrect because a rectifier is not specifically designed for this purpose. A rectifier simply converts AC voltage into DC voltage, regardless of the shape or type of the AC waveform.
Option 4 - None of the above is incorrect because the correct answer is option 1.
In summary, a rectifier is used to convert AC voltage into DC voltage, making option 1 the correct answer in this case.