Final answer:
In 1959, the requirement for missionary appointees included minimal fluency in the prevailing language of the field.
Step-by-step explanation:
In 1959, the requirement for missionary appointees included minimal fluency in the prevailing language of the field. This meant that individuals who wanted to become missionaries had to have a basic understanding of the language spoken in the area they would be working in. This was important for effective communication and engagement with the local population. The other options listed in the question, such as psychological testing, a prescribed course, or specific degrees, were not mentioned as requirements for missionary appointees in 1959.