Final answer:
The Full Scale IQ score is the most reliable score when determining global ability because it encompasses a comprehensive measure of various cognitive domains, aligning with Wechsler's definition of intelligence.
Step-by-step explanation:
When determining global ability, the most reliable score is the Full Scale IQ score. The Full Scale IQ score is derived from a combination of subtests that comprise five indices: Verbal Comprehension, Visual Spatial, Fluid Reasoning, Working Memory, and Processing Speed, each reflecting different cognitive abilities. The Full Scale IQ score is considered the most reliable because it provides a comprehensive overview of an individual's intelligence, taking into account various cognitive domains. The use of the Full Scale IQ is in line with David Wechsler's belief of intelligence as “the global capacity of a person to act purposefully, to think rationally, and to deal effectively with his environment”, which is foundational to the Wechsler scales such as the WAIS-IV and WISC-V.