1. The problem asks: At what age does the human brain reach 50 percent of its adult size? 2. The options given are: 1 month, 6 months, 25 years, and 5 years. 3. According to developmental neuroscience, the human brain reaches about 50% of its adult size by around 6 months of age. 4. This is because brain growth is very rapid in the first year of life, with significant increases in size and complexity. 5. Therefore, the correct answer is 6 months. Final answer: **6 months**.