**Core Concept**
Mouth-to-mouth breathing, also known as rescue breathing, is a technique used in cardiopulmonary resuscitation (CPR) to provide oxygen to a person who has stopped breathing. The goal is to deliver a sufficient amount of oxygen to the respiratory system to maintain tissue oxygenation and prevent brain damage.

**Why the Correct Answer is Right**
During mouth-to-mouth breathing, air is delivered into the subject's lungs through the mouth. The air is then exchanged within the lungs, and oxygen is absorbed into the bloodstream. Studies have shown that mouth-to-mouth breathing can deliver approximately 16% oxygen to the subject's respiratory system. This is because the air we breathe is composed of approximately 21% oxygen, 78% nitrogen, and 1% other gases. When air is breathed into the lungs through mouth-to-mouth resuscitation, the oxygen content is diluted, resulting in a lower concentration of oxygen being delivered to the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not provided, so no explanation can be given.
**Option B:** This option is not provided, so no explanation can be given.
**Option C:** This option is not provided, so no explanation can be given.
**Option D:** This option is not provided, so no explanation can be given.

**Clinical Pearl / High-Yield Fact**
It is essential to note that mouth-to-mouth breathing should only be performed by trained individuals, such as healthcare professionals, due to the risk of transmission of infectious diseases.

**Correct Answer: C. 16%**