Final answer:
John Bunyan, a Puritan thinker and preacher, began writing 'The Pilgrim's Progress' in the 1670s while incarcerated for illegal preaching. This allegorical work became a significant piece of religious English literature, embodying Puritan values and Bunyan's personal spiritual journey.
Step-by-step explanation:
The Puritan thinker who began writing 'The Pilgrim's Progress' while in prison for illegal preaching in the 1670s was A) John Bunyan. Bunyan was incarcerated for conducting religious gatherings outside the auspices of the official Church of England. His influential work, 'The Pilgrim's Progress', is an allegorical tale that charts the journey of a character named Christian as he traverses various challenges and temptations to reach the Celestial City, which represents heaven.
The Pilgrim's Progress was written in two parts, with the first published in 1678 and the second in 1684. This religious allegory reflects Bunyan's own spiritual journey and has been recognized as one of the most significant works of religious English literature, highlighting the Puritan value of piety and perseverance in the face of adversity. Despite being written while the author was imprisoned, the book went on to become a seminal classic, widely read for generations and translated into numerous languages.