Answer:
C. After standing outside for more than an hour on a cold rainy day, the group decided that the bus would never come.
Explanation:
A. After stood outside for more than an hour on a cold rainy day, the group decided that the bus will never come.
needs a pronoun like "they" before the word "stood", and will->would because will is present tense
B. After were standing outside for more than an hour on a cold rainy day, the group decides that the bus won't come.
"After [they] were" "the group decide[d]"
D. After stands outside for more than an hour on a cold rainy day, the group decides that the bus will never come.
"stands" would not be correct here, "standing" would