Spearman inferred a single general factor; Thurstone argued for seven relatively independent abilities. Both were analysing correlations among mental tests. How did they reach opposite conclusions from the same kind of data?

B. Because factor analysis does not deliver one unique answer.

Factor analysis extracts a structure and does not tell you which is correct, an unrotated solution produces a large first factor, and rotating for simple structure spreads the variance. The resolution came from Thurstone's own data: his seven abilities correlated with each other, and factoring those correlations produces something that looks a great deal like g.