**Core Concept**
The umbilical cord undergoes a series of changes after birth, reflecting the gradual cessation of placental function and the establishment of independent circulation in the newborn. One of these changes is the discoloration of the umbilical cord.

**Why the Correct Answer is Right**
The umbilical cord becomes black due to the drying out of the Wharton's jelly, a gelatinous substance that surrounds the two umbilical arteries and one umbilical vein. As the cord dries out, it becomes less vascular and the blood vessels within it undergo thrombosis, leading to the characteristic black coloration. This process typically occurs 1-3 days after birth.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the discoloration of the umbilical cord.
**Option B:** While meconium aspiration can cause discoloration of the lungs, it is not related to the color change of the umbilical cord.
**Option C:** Umbilical cord separation is a separate process from cord discoloration and usually occurs around 1-2 weeks after birth.

**Clinical Pearl / High-Yield Fact**
The timing of umbilical cord discoloration can serve as a rough estimate of the postnatal age of a newborn, with blackening typically occurring 1-3 days after birth.

**Correct Answer: D. 1-3 days after birth.**