What is the difference between a diversifiable risk and a nondiversifiable risk? Should stock portfolio managers try

Question:

What is the difference between a diversifiable risk and a nondiversifiable risk? Should stock managers try to eliminate both types of risk?

Portfolio
A portfolio is a grouping of financial assets such as stocks, bonds, commodities, currencies and cash equivalents, as well as their fund counterparts, including mutual, exchange-traded and closed funds. A portfolio can also consist of non-publicly...