What is the most practical reason for adding black separately in printing?

Correct answer: Cyan, magenta, and yellow do not produce a perfect deep black in practice

The governing concept is practical subtractive printing, represented by CMYK. In theory, cyan, magenta, and yellow can absorb much of the visible spectrum, but real inks are not ideal; their combination often gives a muddy brownish or weak black. A separate black, the K channel, supplies dense shadows, sharper text, better detail, and more economical dark coverage. Thus option A is the practical reason.