**Core Concept**
A tracer bullet is a type of ammunition designed to leave a visible mark or trail in its flight path, allowing the shooter or observers to track its trajectory. This is achieved through the use of a pyrotechnic or incendiary material that burns brightly, creating a visible path.

**Why the Correct Answer is Right**
Tracer bullets contain a pyrotechnic or incendiary material, typically a burning pyrophoric metal such as magnesium or a pyrotechnic composition, which is embedded in the bullet's base. As the bullet travels, the burning material creates a bright, visible trail that can be seen by the shooter or observers. This allows for easier tracking of the bullet's trajectory, making it a useful tool for military and law enforcement applications.

**Why Each Wrong Option is Incorrect**
* **Option A:** A tandem bullet is a type of bullet that has a separate projectile attached to the base of the primary bullet, designed to separate and continue on its own trajectory. Tandem bullets do not leave a visible mark in their flight path.
* **Option C:** A Dum-dum bullet is a type of expanding bullet that is designed to expand upon impact, increasing its diameter and causing more damage to the target. Dum-dum bullets do not leave a visible mark in their flight path.
* **Option D:** An incendiary bullet is a type of bullet designed to start fires upon impact, typically used for military or law enforcement applications. While incendiary bullets can cause fires, they do not leave a visible mark in their flight path.

**Clinical Pearl / High-Yield Fact**
When analyzing bullet trajectories in forensic medicine, it is essential to consider the type of ammunition used, as different types of bullets can leave distinct marks or trails. Tracer bullets, in particular, can create a visible path that can be used to reconstruct the crime scene.

✓ Correct Answer: B. Tracer bullet