The correct answer is: C. Either I or II is implicit
The statement “Lack of stimulation in the first four or five years of life can have adverse consequences” suggests that early childhood development is important. This is consistent with both assumptions I and II. Assumption I states that a great part of the development of observed intelligence occurs in the earliest years of life. Assumption II states that 50 percent of the measurable intelligence at age of 17 is already predictable by the age of four. Both of these assumptions suggest that early childhood development is important and that lack of stimulation during this time can have negative consequences.
However, the statement does not explicitly state that either assumption is true. It is possible that there are other factors that are also important for early childhood development, or that lack of stimulation does not always have negative consequences. Therefore, we cannot say for certain that either assumption is true, but we can say that they are both consistent with the statement.