Final answer:
Milton was imprisoned for his association with the Commonwealth government and his defense of the regicide of Charles I, not for religious or financial reasons.
Step-by-step explanation:
The reason Milton was imprisoned was due to his role as the Latin secretary for the government of Oliver Cromwell during the Commonwealth period. After the restoration of the monarchy under Charles II, Milton was arrested for his involvement with the Commonwealth government and his defense of the execution of Charles I. His incarceration was not due to a denial of God's mercy, refusal to write for Charles II, or financial debts to his father-in-law.