According to the ACAMS CAMS Certification Study Guide (6th edition), the bank should file a SAR if it knows, suspects, or has reason to suspect that a transaction involves funds derived from illegal activity, or is intended or conducted to hide or disguise funds or assets derived from illegal activity, or to evade any BSA regulation or federal law, or has no business or apparent lawful purpose, or is not the sort in which the customer would normally be expected to engage1. The scenario described in the question meets these criteria, as the small deposits and withdrawals from a high-risk country could indicate money laundering, terrorist financing, or other illicit activities. The bank should also document its decision to file or not file a SAR, and retain the supporting documentation for five years1.
The other options are not correct because they either do not comply with the BSA requirements, or do not adequately address the potential risk of the activity. Blocking any further activity could alert the customer of the bank's suspicion, and could also interfere with law enforcement investigations. Initiating an investigation into the activity could be part of the bank's due diligence process, but it does not substitute the obligation to file a SAR if the activity is suspicious. Contacting the customer could also tip off the customer, or elicit false or misleading explanations that could hinder the bank's assessment of the activity.
References: ACAMS CAMS Certification Study Guide (6th edition), page 82-83. 12345678 [9]

Explanation
Explanation/Reference:
The Integration Stage
The final stage of the money laundering process is termed the integration stage. It is at the integration stage where the money is returned to the criminal from what seem to be legitimate sources. Having been placed initially as cash and layered through a number of financial transactions, the criminal proceeds are now fully integrated into the financial system and can be used for any purpose. There are many different ways in which the laundered money can be integrated back with the criminal; however, the major objective at this stage is to reunite the money with the criminal in a manner that does not draw attention and appears to result from a legitimate source. For example, the purchases of property, art work, jewellery, or high-end automobiles are common ways for the launderer to enjoy their illegal profits without necessarily drawing attention to themselves.

The following were the key features of the Second Directive:
* It extended the scope of the First Directive beyond drug-related crimes. The definition of "criminal activity" was expanded to cover not just drug trafficking, but all serious crimes, including corruption and fraud against the financial interests of the European community.
* It explicitly brought bureaux de change and money remittance offices under AML coverage.
* It clarified that knowledge of criminal conduct can be inferred from objective factual circumstances.