C

The overall injury rate in early childhood rises sharply once toddlers begin walking independently and peaks in the 15-to-17-month window, squarely within the 12-15-month age range. Large population data show the highest hospitalization/death rate for injuries at 15-17 months among children ≤3 years old [Agran, 2003, PMID 12777586]. This corresponds to the developmental stage when mobility and exploration are high while hazard awareness is low, making 12-15 months the period with the greatest risk of injury.