Hermann ebbinghaus measured forgetting by: a) measuring how long it took to relearn a previously learned list. b) measuring how long it took him to learn a list perfectly. c) seeing how long a list he could remember after hearing the list just once. d) counting the number of times he needed to hear a list before he could recall it perfectly.

C) seeing how long a list he could remember after hearing the list just once.

Explanation:

Herman Ebbinghaus has developed a list of non-sense syllables which he tried to learn and then tested himself if he could retain the words after different point of time. He believed that memory retention is 100% at the time of remembering the list and then gradually decreases by 40% after the first few days.