I believe it is option 2. (a chronological book of American history)
A; therefore would be the best transition to fit the blank
The theory of plate tectonics states that the Earth's surface, the upper mantle and crust, was once made up of enormous rock plates that broke into smaller pieces approximately 300 million years ago. These smaller, broken plates form a more fluid rock surface in the mantle. Over time, the plates move and morph into natural land boundaries. This explains natural land phenomena, such as earthquakes and tsunamis.So A
The indefinite pronoun is somebody.
