As medical knowledge improved beginning in the 18th century, psychological disorders became a. more cloistered b. less prevalent c. less mysterious d. further stigmatized

The answer is 'C'(less mysterious)

Mental disorder would designs for behavioral or mental side effects that sway numerous zones about an aggregation.

In the beginning of eighteenth century improved medical knowledge in this way the medical science,surgery and diagnosis are also improved.
  Psychological disorder remain not  a big issue it became doubtless as the treatment of psychological disorder is become powerful.