White-tailed deer populations have exploded to approximately 20-30 million today due to habitat changes, hunting regulations, and lack of natural predators. This represents a 40-60x increase from 1900 levels. Conservation efforts, hunting restrictions, and suburban expansion created ideal deer conditions. The population growth has been so significant that deer are now considered overpopulated in many regions, causing ecological damage and vehicle collisions. The 20 million figure (Option D) best reflects this dramatic recovery and overabundance.