**Core Concept:** Capillary filling refers to the process of red blood cells (RBCs) and plasma filling the capillaries and maintaining the blood flow within them. Capillary beds are essential for proper tissue perfusion, waste removal, and nutrient delivery.

**Why the Correct Answer is Right:** The correct answer, **Option D:** umbilical cord, is the ideal site to demonstrate capillary filling in infants. In infants, the umbilical cord remains connected to the placenta until birth, making it an essential part of the circulatory system during fetal development. As the placenta supplies the fetus with oxygen and nutrients, it ensures proper capillary filling and blood flow within the umbilical cord itself. Additionally, the blood flow in the umbilical cord is under the control of the placenta, allowing for the assessment of capillary filling during fetal development.

**Why Each Wrong Option is Incorrect:**

A. **Option A (Nasal mucosa):** While the nasal mucosa is an essential site for capillary filling in adults, its capillaries are superficial and interconnected, making them difficult to visualize in infants.

B. **Option B (Digital mucosa):** Similar to the nasal mucosa, the capillaries in the digital mucosa are superficial and interconnected, rendering them unsuitable for demonstrating capillary filling in infants.

C. **Option C (Liver):** The liver is an organ with a complex architecture, including sinusoids and parenchyma, which makes it difficult to visualize capillary filling directly. Additionally, assessing capillary filling in the liver is challenging in infants due to its depth and the presence of a fibrous capsule.

**Clinical Pearl:** The umbilical cord provides an opportunity to observe capillary filling in infants, enabling medical professionals to evaluate fetal development and assess the efficiency of placental function.

**Correct Answer:** Option D (Umbilical cord)