A review of Internet security disclosed that users have individual user accounts with Internet service...

The first step that the organization should take to ensure that only the corporate network is used for downloading business data is to include a statement in its security policy about Internet use. A security policy is a document that defines the rules, expectations, and overall approach that an organization uses to maintain the confidentiality, integrity, and availability of its data1. A security policy should clearly state the acceptable and unacceptable use of Internet resources, such as personalaccounts with ISPs, and the consequences of violating the policy. A security policy also helps to guide the implementation of technical controls, such as proxy servers, firewalls, or monitoring tools, that can enforce the policy and prevent or detect unauthorized Internet access.

The other options are not the first step that the organization should take, but rather subsequent or complementary steps that depend on the security policy. Using a proxy server to filter out Internet sites that should not be accessed is a technical control that can help implement the security policy, but it does not address the root cause of why users are using personal accounts with ISPs. Keeping a manual log of Internet access is a monitoring technique that can help audit the compliance with the security policy, but it does not prevent or deter users from using personal accounts with ISPs. Monitoring remote access activities is another monitoring technique that can help detect unauthorized Internet access, but it does not specify what constitutes unauthorized access or how to respond to it.

References:

ISACA CISA Review Manual 27th Edition (2019), page 247

What is a Security Policy? Definition, Elements, and Examples - Varonis1