Final answer:
Nineteenth-century German researchers in psychophysics used psychophysical methods to study sensory perception, as brain imaging technologies like EEG, MRI, and fMRI were not yet developed.
Step-by-step explanation:
German researchers in psychophysics from the nineteenth century tested their hypotheses using D) Psychophysical methods. During that time, the technology for brain imaging like electroencephalography (EEG), magnetic resonance imaging (MRI), or functional magnetic resonance imaging (fMRI) did not exist. Instead, they relied on various experimental methods to understand the relationships between stimuli in the environment and the sensory experiences of human beings. Psychophysical methods typically involved measuring the thresholds at which stimuli are perceived and systematically varying these stimuli to understand perception.