**Core Concept**
The question is testing the understanding of the family life cycle, specifically the stage of contraction of the family. This refers to a period when family size decreases, usually due to children growing up and leaving the family home. It is a significant phase in the family life cycle, characterized by changes in family dynamics and relationships.

**Why the Correct Answer is Right**
The correct answer, "Leaving home of first child," marks the beginning of the contraction stage. This occurs when the first child reaches adulthood and becomes independent, often by moving out of the family home. As more children leave the family, the family size decreases, and the family enters a new phase of contraction. This stage is critical in understanding family development and planning for social and preventive medicine interventions.

**Why Each Wrong Option is Incorrect**
**Option A:** Birth of first child marks the beginning of the expansion stage, not contraction. It is a time of increased family size and energy expenditure.
**Option B:** Birth of last child is usually followed by a period of expansion, as the family continues to grow and adapt to new additions.
**Option D:** Marriage is a significant life event that often marks the beginning of the expansion stage, as a new family unit is formed.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that family life cycles are dynamic and influenced by various factors, including cultural, social, and economic contexts. Understanding these cycles is crucial for developing effective family-centered health interventions and policies.

**✓ Correct Answer: C. Leaving home of first child**