D is the answer to this question. You should first understand what SAR means. This stands for Suspicious Activity Reporting. This means that if you detect that there are some inconsistencies with the facts that you have received so far, you can file within 30 days.
This is the reason why D is the answer instead of B. If in case there is no suspect that is identified for the activity that has been reported, it will be extended for up to 60 days. In this case, you have already identified that it is the foreign client who may have undeclared money in his account. The other agencies are meant to handle other issues.