Final answer:
The scientists were mainly happy when the monkeys started working on the puzzles on their own. So the correct answer is Option B.
Step-by-step explanation:
When the monkeys started working on the puzzles on their own, the scientists were mainly happy. The reference text mentions that the work with Koko provided a wealth of data and opened up new possibilities for understanding the language abilities of nonhuman primates. This implies that the scientists were pleased with the monkeys' progress in solving puzzles and the data they were able to gather.