**Core Concept**
The question is testing the ability to identify a condition that requires further investigation in a neonate, specifically focusing on the clinical presentation and age of the baby.

**Why the Correct Answer is Right**
Babies between 3-5 days old presenting with different complaints may have a condition that requires immediate attention. One such condition is **Neonatal Hypoglycemia**. Neonatal hypoglycemia occurs when the blood glucose levels drop below 40 mg/dL in the first few days of life. This condition can be caused by a variety of factors such as maternal diabetes, polycythemia, or congenital disorders. Hypoglycemia can lead to neurological damage, seizures, and even death if left untreated.

**Why Each Wrong Option is Incorrect**
* **Option A:** A 3-day-old baby with a normal temperature and no signs of infection does not necessarily require further investigation.
* **Option B:** A 4-day-old baby with a fever and a normal white blood cell count may be a case of viral infection, which may not require further investigation unless the symptoms worsen.
* **Option C:** A 5-day-old baby with a normal physical examination and no complaints does not require further investigation unless there are any specific concerns or risk factors.

**Clinical Pearl / High-Yield Fact**
Always consider neonatal hypoglycemia as a potential diagnosis in any baby between 3-5 days old presenting with different complaints, especially if there is a history of maternal diabetes or polycythemia.

**Correct Answer: C. A 5-day-old baby with a normal physical examination and no complaints.**