Final answer:
The case study of Gina, also known as Genie, is most fascinating for showcasing the role of language in cognitive development and the detrimental impact isolation can have on this process.
Step-by-step explanation:
The case study of Gina the wild child or Genie, as she is often known, has provided researchers with invaluable insights into the impact of isolation on human development. The most fascinating aspect, in this context, is the role of language in cognitive development. Genie's unique situation showed how critical early social interactions and language exposure are to a child's development. Her inability to master grammar despite developing an impressive vocabulary suggests a critical period in language development. Without the early socialization experiences, including exposure to language, children may struggle to fully develop cognitive abilities, as demonstrated by Gina's improved abilities after being removed from isolation.