Answer:
D: Magnetism
Step-by-step explanation:
Separating two substances in a mixture by hand can be tedious and is often expensive due to the time it takes to sort through each individual piece. A good example is separating iron fillings from saw dust.
Large companies would often need to sort through large amounts of mixtures. Hence it would be easier to use a magnet since iron fillings are attracted to magnets and saw dust is not.