Which turning point would best start an era called "westward expansion"?

Hmm, there were several triggering events that promoted westward expansion of the United States.  Victory in war over Mexico (1848) gave the US new lands in the west.  A treaty with Britain (also in 1848) gave the US sole possession of some formerly disputed Oregon territory lands also. Plus, in 1848, gold was discovered in California (land newly in US possession after that Mexican war treaty), so that prodded westward expansion too.  Perhaps those are the initial "turning points" you're looking for. The completion of the Transcontinental Railroad in 1869 sure helped the process as it was underway, as well.

I’m pretty sure that the answer is b.