Final answer:
The phrase 'Every contact leaves a trace' best summarizes Locard's exchange principle, which is a key concept in forensic science illustrating that contact between objects results in an exchange of material.
Step-by-step explanation:
The phrase that best sums up Locard's exchange principle is A. "Every contact leaves a trace." This principle is a foundational concept in forensic science, which was formulated by Dr. Edmond Locard, a pioneer in the field. The principle posits that whenever two objects come into contact, there's an exchange of materials between them. For example, a criminal at a crime scene may leave behind trace evidence such as fingerprints, hair, or fabric fibers, and take away some evidence from the scene on their clothing or shoes.