**Core Concept**
The size of an endotracheal tube (ETT) in a newborn is determined by the weight of the baby and is used to prevent complications such as subglottic stenosis or bronchial intubation. The internal diameter of the ETT is proportional to the weight of the baby, typically using a formula or nomogram to guide selection.

**Why the Correct Answer is Right**
For a newborn weighing 2.5 kg, a 3.0 mm internal diameter ETT is the correct size. This is because the size of the ETT is directly proportional to the weight of the baby, and a 2.5 kg baby requires a slightly larger ETT to ensure adequate ventilation and prevent complications. The internal diameter of the ETT is critical in maintaining a patent airway and preventing over- or under-ventilation.

**Why Each Wrong Option is Incorrect**
**Option A:** 2.5 mm is too small for a 2.5 kg newborn, as it may lead to inadequate ventilation and increased risk of complications.
**Option B:** 4.0 mm may be too large for a 2.5 kg newborn, as it may cause over-ventilation and increase the risk of lung injury.
**Option C:** 3.5 mm is not a standard size for a newborn ETT, and using an non-standard size may increase the risk of complications.

**Clinical Pearl / High-Yield Fact**
When selecting an ETT for a newborn, it is essential to use a size that is proportional to the baby's weight to prevent complications such as subglottic stenosis or bronchial intubation.

**Correct Answer: C. 3.0 mm**