In the given sentence, “is” depicts past tense, “was” depicts past tense. Therefore, the pack is a singular and wolves are plural. However, “the pack of wolves” is considered as singular.
Therefore, “is” is correct grammatical preset tense used when they are running through the forest. Other words “was” can’t be used in this case.
Thus, the options (d), (b) and (a) are incorrect.
Therefore, “is” is correct grammatical preset tense used when they are running through the forest. Other words “was” can’t be used in this case.
Therefore, the correct option is – c. is.