To determine which geographic feature escaping enslaved people would need to cross to gain their freedom on the Underground Railroad, it is essential to consider the routes typically taken. The Rio Grande (D) is a significant river that forms part of the border between the United States and Mexico, which escaping enslaved people might cross to reach freedom in Mexico. Other options like the Brazos River, Red River, and Colorado River are less significant in the context of the Underground Railroad.
Thus, based on the context, the answer would be:
D. The Rio Grande