Donders timed mental stages by subtracting reaction times

Correct: B — Reaction-time differences between tasks reflect the time of inserted mental stages

Donders compared three tasks: simple RT (one stimulus, one response), choice RT (multiple stimuli and responses), and go/no-go (multiple stimuli, one response). He inferred that the difference between simple and go/no-go isolates "stimulus discrimination" time, and the further difference to choice RT isolates "response selection." The subtraction method assumes pure insertion — adding a stage does not change other stages — a strong assumption that Sternberg's additive-factors logic (1969) later refined. Donders is the founding paper of mental chronometry.