Answer:
It's C.
Step-by-step explanation:
Had arrived meant they are already there.
Arrives is for SINGULAR nouns, not ones that are already plural. They is plural. Therefore, the letter 'S' shouldn't be there.
Arrived is saying the present. They arrived now.
Nonetheless, C says it all because...
1) It had no letter 'S' to make the sentence more plural then it already is.
2) It's talking about the future, not the past nor present.
Hope this helps!!