Final answer:
The roots of ethical subjectivism in romantic sentimentalism developed in the 19th century.
Step-by-step explanation:
The roots of ethical subjectivism in romantic sentimentalism can be traced back to the 19th century. Romanticism, as a literary and artistic movement, emphasized the value of individuality and subjectivity, shifting the focus from outward action to inward action. It valued emotions over logic and nature over industry, and it rejected the scientific and industrial ideals of the time. Therefore, option a) developed in the 19th century is the correct answer.