The text identifies three categories of people who are good professional references: work colleagues, former employers, and

The text identifies three categories of people who are good professional references: work colleagues, former employers, and "former instructors or school advisers".

A professional reference means a suggestion from someone who can vouch for your capabilities and qualities for work. A professional reference for an accomplished worker is commonly a previous manager, a partner, a customer, a director, or another person who can prescribe you for business.

Old telephone. is prehistoric object