**Core Concept**
Adenoids, also known as the nasopharyngeal tonsils, are a mass of lymphoid tissue located in the back of the nasal cavity. They play a crucial role in the immune system, particularly in children, by filtering out pathogens and other foreign substances from the air we breathe.

**Why the Correct Answer is Right**
Adenoids are most commonly diagnosed using a physical examination and a mirror or endoscope to visualize the nasopharynx. The correct answer is likely related to a physical examination technique or finding associated with adenoid pathology.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while imaging studies like X-rays or CT scans can be used to evaluate adenoid size and position, they are not the primary method for diagnosing adenoids.

**Option B:** This option is incorrect because while adenoiditis, or inflammation of the adenoids, is a common condition, it is not the most objective test for adenoids.

**Option C:** This option is incorrect because while nasal endoscopy can be used to visualize the adenoids, it is not the most objective test for adenoids.

**Option D:** This option is incorrect because while adenoidectomy, or removal of the adenoids, is a common surgical procedure, it is not a diagnostic test.

**Clinical Pearl / High-Yield Fact**
A key clinical correlation to remember is that adenoid hypertrophy is a common cause of recurrent otitis media (middle ear infections) in children.

**Correct Answer: D. Adenoidectomy is the definitive treatment of choice for adenoid hypertrophy.**