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They are embedded in the constitution
"The... shall choose their other Officers, and also a President pro tempore, in the absence of the Vice President, or when he shall exercise the Office of President of the United States."
They were all slave states that had remained in the Union and this is what Missouri, Kentucky, Maryland and Delaware had in common during the Civil war. The correct option among all the options that are given in the question is the second option. I hope that this is the answer you were looking for and it helped you.
