The correct answer is- while running through​. Key Points​ ​Let's understand the difference between the adverbs given below: When- at or on which (referring to a time or circumstance) While- during the time that; at the same time as We use 'while' with two continuous actions and 'when' with two single actions. Example: He was playing while I was studying. (two continuous actions) I picked up the phone when it rang.​ (two single actions) Since two events are taking place simultaneously (the mare getting hurt and running), 'while' will be used in the underlined part. Correct Sentence: The mare hurt its hooves while running through the forest. ​​​​Additional Information ​​ ​Let's understand the difference between the adverbs given below: Hard- With a great amount of effort. Hardly- scarcely (used to qualify a statement by saying that it is true to an insignificant degree). For example: He hard does any work.[ alt="false (2)" src="https://storage.googleapis.com/tb-img/production/20/08/false%20%282%29.png" style=" white-space: pre-wrap; width: 25px; height: 25px;"> He hardly does any work.[ alt="right (1)" src="https://storage.googleapis.com/tb-img/production/20/08/right%20%281%29.png" style="width: 25px; height: 25px;">