Final answer:
The Gulf War in 1990-1991 is the event that involved a U.S. president using singular military power to enter into a conflict. The other options either do not involve military conflict or do not involve initiating conflict.
Step-by-step explanation:
The event that signifies a U.S. president using singular military power to enter into a conflict is c) The Gulf War in 1990-1991. Though the other options did involve presidents taking significant actions, none of the examples constitute a direct use of singular military power to enter a conflict. The signing of the Emancipation Proclamation by Abraham Lincoln was more of an executive action altering the status of enslaved people during the on-going Civil War rather than an initiation of a conflict. The Affordable Care Act is about health policy, and the construction of the Transcontinental Railroad is about infrastructural development. In contrast, The Gulf War was a military conflict initiated under President George H.W. Bush's administration in response to Iraq's invasion of Kuwait.
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