**Core Concept**
Physiological umbilical hernia, also known as a patulous or patent urachus, is a benign condition where the urachus, a fibrous remnant of the allantois, fails to close completely during fetal development. This results in a small protrusion of the umbilicus, particularly when intra-abdominal pressure increases.

**Why the Correct Answer is Right**
As intra-abdominal pressure decreases, such as during relaxation or after a meal, the urachus collapses, reducing the hernia. Conversely, when intra-abdominal pressure increases, such as during straining, coughing, or heavy lifting, the urachus dilates, causing the hernia to protrude. This phenomenon is due to the continuous communication between the bladder and the umbilicus through the urachus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option might suggest that the reduction of physiological umbilical hernia is due to the contraction of the abdominal muscles, which is not the primary mechanism. While abdominal muscle contraction can momentarily decrease intra-abdominal pressure, it does not address the underlying patency of the urachus.
**Option B:** This option might imply that the reduction is due to the closure of the urachus, which is not accurate. The urachus remains patent in physiological umbilical hernia.
**Option C:** This option is vague and does not provide a clear explanation for the reduction of physiological umbilical hernia. The correct answer lies in the relationship between intra-abdominal pressure and the patency of the urachus.

**Clinical Pearl / High-Yield Fact**
Physiological umbilical hernia is often asymptomatic and may be discovered incidentally during a routine examination. However, it can cause discomfort or pain if it becomes incarcerated or strangulated, which is a medical emergency.

**Correct Answer:** C.