Final answer:
Charles Spearman is the theorist most likely to argue that intelligence is a single, innate quality measurable by standardized intelligence tests, due to his g factor theory. The Wechsler IQ tests are the most widely used today.
Step-by-step explanation:
The theorist most likely to argue that intelligence is a single, innate quality that can be measured by standardized intelligence tests is Charles Spearman. Spearman proposed the theory of general intelligence, also known as the g factor, which suggests that one general factor underlies all intelligent behavior. This is in contrast to theories such as Howard Gardner's Multiple Intelligences Theory, which posits that there are several distinct forms of intelligence.
On the topic of IQ testing, the IQ test most widely used today was not developed by Sir Francis Galton, Alfred Binet or Louis Terman; it was developed by David Wechsler. Wechsler's contributions to intelligence testing allowed for a broader understanding and measurement of intelligence beyond just verbal capabilities, taking into account both verbal and nonverbal elements.