**Core Concept**
The term "yawning" bullet refers to a type of bullet that is designed to expand upon impact, causing a larger wound diameter than the original bullet size. This expansion can result in a characteristic "keyhole" or "mousehole" entry wound, which is often seen in forensic pathology.

**Why the Correct Answer is Right**
The yawning bullet is designed to expand upon impact, causing the bullet to mushroom and increase in diameter. This expansion creates a larger wound diameter than the original bullet size, resulting in a characteristic "keyhole" entry wound. The expansion of the bullet is due to the deformation of the bullet's core and the subsequent compression of the surrounding tissue. This type of wound is often seen in forensic pathology and can be an important clue in determining the type of firearm used in a crime.

**Why Each Wrong Option is Incorrect**
**Option A:** A tracer bullet is designed to leave a visible mark or trail when fired, but it does not expand upon impact and is not typically associated with keyhole entry wounds.

**Option B:** A tandem bullet is a type of bullet that consists of two bullets joined together, but it is not designed to expand upon impact and is not typically associated with keyhole entry wounds.

**Option C:** A Dum-Dum bullet is a type of bullet that is designed to expand upon impact, but it is typically associated with a larger, more irregular wound, rather than a keyhole entry wound.

**Clinical Pearl / High-Yield Fact**
In forensic pathology, the type of bullet used in a crime can be an important clue in determining the identity of the firearm used. By examining the characteristics of the wound, such as the size and shape, investigators can often determine the type of bullet used and the firearm from which it was fired.

**✓ Correct Answer: D. Yawning bullet. A type of bullet that expands upon impact, resulting in a characteristic "keyhole" entry wound.**