By the late 1610s, what religious group had been persecuted, fled to Holland, then finally established the colony of Plymouth?

Puritans.

Explanation:

Puritans' religion emerged at the time of English Reformation during the sixteenth century. Puritans held belief that the Church of England, like Roman catholic Churches, include practices and ceremonies that are not in the Bible.

In the early seventeenth century, Puritans separated themselves from the local churches. In 1610, some group of Puritans fled to Holland for the sake of their safety. In 1620, they moved to New England and established colonies for themselves in Plymouth. These group of people are now known as Pilgrims of Plymouth Rock.

So, the correct answer is "Puritans".