Which psychophysicist's major contribution was to demonstrate that the relationship between incoming stimuli and corresponding perceptions was not one-to-one?

The psycho-physicist we are talking about here is Fechner.

Thus,

the answer is none other than Gustav Fechner.

Gustav Theodor Fechner was indeed the founder of the experimental psychology. This experimental psychology is the one that applied the scientific method to the research of behavior and mind. And the question above is also one of the contributions of Gustav Fechner and his works.

there are no options provided but you can choose among the following gender-neutral pronouns: zir, hir, eir, vis, tem, eir.

in the normal course of maintaining a pronoun-antecedent agreement, we could have used the words his or her but since we are also required to use gender-neutral language, we need to replace it with its equivalent gender-neutral pronoun.