Economic sanctions are penalties imposed by one or more countries against another country, group, or individual for violating international norms or threatening national interests1. The ultimate goal of economic sanctions is to protect national security by changing the behavior or policies of the target, or by weakening its capabilities or resources2. Sanctions can be used to advance various foreign policy objectives, such as counterterrorism, nonproliferation, democracy promotion, human rights protection, conflict resolution, and cybersecurity1. Sanctions can take different forms, such as travel bans, asset freezes, trade embargoes, arms restrictions, and aid reductions1. Sanctions can be applied unilaterally by one country, or multilaterally by a coalition of countries or an international organization, such as the United Nations or the European Union1.
References:
1: What Are Economic Sanctions? | Council on Foreign Relatio
2: How Economic Sanctions Work - Investopedia
Reference: https://www.investopedia.com/articles/economics/10/economic-sanctions.asp

According to the USA PATRIOT Act, a shell bank is defined as "a bank that has no physical presence in any country" (Section 313(a)(1)). A physical presence means "a place of business that is maintained by a bank and is located at a fixed address, other than solely a post office box or an electronic address, in a country in which the bank is authorized to conduct banking activities, at which location the bank employs one or more individuals on a full-time basis and maintains operating records related to its banking activities" (Section
313(a)(2)). Therefore, option A is the correct answer, as it describes a bank that has no physical presence or employees in any country. Options B, C, and D are not correct, as they describe banks that have some form of physical presence or affiliation with another bank in a country.
References:
USA PATRIOT Act, Title III, Subtitle A, Section 313, Prohibition on United States Correspondent Accounts with Foreign Shell Banks, 1.
ACAMS Study Guide for the Certified Anti-Money Laundering Specialist (the 6th edition), Chapter 4:
International Standards and Global Initiatives, page 103.