What were the first experimental investigations on memory and who conducted them?

Hermann Ebbinghaus studied how syllables without meaning are retained and defended that the memory mechanism requires a single activity (repetition) for the data we remember to be associated with each other.
Frederic Barlett studied memory using stories and introduced the theory of schemas and their influence on memories.
George Miller demonstrated that people can retain approximately five to seven elements at a time in short-term memory.
a) Only I is correct.
b) Only II is correct.
c) Only III is correct.
d) I, II, and III are correct.