Final answer:
Stonehenge's initial construction as a simple earthwork enclosure began around 3100 BCE, with more elaborate structures like stone circles added by 2400 to 2200 BCE.
Step-by-step explanation:
The first part of Stonehenge was completed around 3100 BCE, marking the beginning of its construction. Initially, it started as an earthwork enclosure with a circular ditch and an external bank. Over time, more complex structures were added, including a circle of standing stones. According to archaeological evidence and radiocarbon dating, the first bluestones in the innermost ring of Stonehenge were erected between 2400 and 2200 BCE. Thus, the correct answer to when the first part of Stonehenge was completed is option c, about 1900 B.C.