Final answer:
Overlanders shifted to the north side of the Platte River and crossed the Continental Divide at South Pass to avoid difficult terrain, utilizing this wider and safer passage along the Oregon Trail. The correct answer is option 1).
Step-by-step explanation:
The reason for overlanders to shift to the north side of the Platte and cross the Continental Divide at the South Pass was to avoid difficult terrain. The South Pass was a critical point along the Oregon Trail due to its low elevation and broad topography, making it one of the few places where wagons could safely traverse the Rocky Mountains. This route helped pioneers avoid the hardships and delays associated with crossing more treacherous parts of the Rockies.