Which of these is thought to have been an ancient observatory?

Research and excavations have revealed that Stonehenge was built in phases, beginning perhaps as early as 3100 BC, and reaching a final form around 1800 BC. Its design seems to correspond to the observation of many astronomical events such as solstices, eclipses, moon cycles, and more. Some theories of alignments are widely accepted. Others are controversial because they could have developed by chance.

Source: Western Washington University