**Core Concept:** Nursing bottle caries, also known as early childhood caries (ECC), is a severe form of dental caries (tooth decay) affecting infants and young children primarily due to prolonged exposure to sugary liquids in baby bottles. This condition predominantly affects the primary (baby) teeth.

**Why the Correct Answer is Right:** The correct answer, **Option D: 12-15 months**, refers to the age group most susceptible to nursing bottle caries. At this stage, the primary teeth (also known as milk teeth) become more prone to dental caries due to increased sugar consumption, prolonged bottle use, and reduced saliva flow that occurs as children become more mobile and independent.

**Why Each Wrong Option is Incorrect:**

A. **1-3 months:** This option is incorrect because the primary teeth start eruption at around 6 months and are fully developed by 30 months, making it too early for nursing bottle caries to occur.

B. **6-9 months:** While this age range is relevant to the initiation of primary teeth eruption, it does not address the specific factors that lead to nursing bottle caries, such as prolonged bottle use and sugar consumption.

C. **3-5 years:** This option represents a broader age group, which does not specifically target the age group most vulnerable to nursing bottle caries.

D. 12-15 months: This is the correct answer as it highlights the age group most susceptible to nursing bottle caries due to increased sugar consumption, prolonged bottle use, and reduced saliva flow at this stage.

**Clinical Pearl:** To prevent nursing bottle caries, parents should avoid giving sugary liquids to infants during sleep or when they are unable to clear sugars from their mouths. Additionally, using a straw for feeding sugary liquids can decrease the risk of caries, as the sugars are mostly directed towards the back teeth, which are less prone to decay.

**Correct Answer:** 12-15 months