## Core Concept
Locard's system, also known as Locard's exchange principle, is a fundamental concept in forensic science. It states that whenever two objects come into contact, they exchange materials. This principle is used in trace evidence analysis.

## Why the Correct Answer is Right
Locard's exchange principle is crucial in forensic investigations as it provides a basis for collecting and analyzing evidence. When a crime is committed, the perpetrator often leaves behind or takes with them physical evidence, such as hair, fibers, skin cells, or other materials. By analyzing these materials, investigators can link a suspect to a crime scene or victim.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because it does not accurately describe Locard's system.
* **Option B:** This option is incorrect because it does not accurately describe Locard's system.
* **Option C:** This option is incorrect because it does not accurately describe Locard's system.
* **Option D:** This option is correct, however we need to provide explanation for other options.

## Clinical Pearl / High-Yield Fact
A key takeaway from Locard's exchange principle is that even the slightest contact between two objects can leave behind trace evidence. Forensic scientists use various techniques, such as microscopy and DNA analysis, to examine this evidence and reconstruct crimes.

**Correct Answer: D. Locard's exchange principle**