While all the options can help reduce the risk of data leakage, providing education and guidelines to employees on the use of social networking sites would be the most effective. This is because it directly addresses the issue at hand - the use of social networking sites for business purposes1. Education and guidelines can help employees understand the risks associated withsocial media use and teach them how to safely and responsibly use these platforms for business purposes1. This includes understanding privacy settings, recognizing phishing attempts, and knowing what information should not be shared on these platforms1.
References:
10 Social Media Guidelines for Employees in 2023 - Hootsuite

The most important reason to classify a disaster recovery plan (DRP) as confidential is to reduce the risk of data leakage that could lead to an attack. A DRP contains sensitive information about the organization's IT infrastructure, systems, processes, and procedures for recovering from a disaster. If this information falls into the wrong hands, it could be exploited by malicious actors to launch targeted attacks, sabotage recovery efforts, or extort ransom. Therefore, a DRP should be protected from unauthorized access, disclosure, modification, or destruction.
The other options are not as important as reducing the risk of data leakage that could lead to an attack:
Ensuring compliance with the data classification policy is a good practice, but it is not a sufficient reason to classify a DRP as confidential. The data classification policy should reflect the level of risk and impact associated with each type of data, and a DRP should be classified as confidential based on its potential harm if compromised.
Protecting the plan from unauthorized alteration is a valid concern, but it is not a primary reason to classify a DRP as confidential. A DRP should be protected from unauthorized alteration by implementing access controls, audit trails, version control, and change management processes. Classifying a DRP as confidential may deter some unauthorized alterations, but it does not prevent them.
Complying with business continuity best practice is a desirable goal, but it is not a compelling reason to classify a DRP as confidential. Business continuity best practice may recommend classifying a DRP as confidential, but it does not mandate it. The decision to classify a DRP as confidential should be based on a risk assessment and a cost-benefit analysis.