The correct option for the most cost-effective and secure way of transmitting confidential messages among the employees of an organization is C. E-mail. Detailed Explanation

1. Understanding the Context:
2. In a professional environment, transmitting confidential messages securely is crucial. Organizations need to ensure that sensitive information is protected from unauthorized access while also being cost-effective.
3. Why E-mail is the Correct Answer:
4. Cost-Effectiveness:
   • E-mail systems are generally inexpensive to set up and maintain. Most organizations already have an e-mail infrastructure in place, which means there are minimal additional costs involved in using e-mail for communication.
5. Security Features:
   • E-mail can be secured using various encryption methods (like TLS for data in transit and PGP for end-to-end encryption). This means that even if the e-mail is intercepted, the content remains unreadable to unauthorized users.
   • Many e-mail services offer additional security features such as two-factor authentication, spam filtering, and phishing protection, which further enhance security.
6. Accessibility and Convenience:
   • E-mails can be accessed from various devices (computers, smartphones, tablets), making it easy for employees to communicate from anywhere.
7. Record Keeping:
   • E-mails provide a written record of communications, which can be important for accountability and reference.
8. Why the Other Options are Incorrect or Weaker:
9. A. Electronic Notice Boards:
   • While electronic notice boards can be used for internal communication, they are not secure for transmitting confidential messages. They are typically accessible to all employees and do not provide privacy for sensitive information.
10. B. Teleconferencing:
    • Teleconferencing is useful for real-time discussions but lacks the ability to securely transmit written confidential messages. Additionally, it may not be cost-effective for large groups or frequent meetings, and it does not provide a permanent record of the conversation.
11. D. Social Networks:
    • Social networks are designed for public sharing and are not secure for confidential communications. They expose information to a wider audience and lack the necessary privacy controls that e-mail systems provide. Using social networks for confidential messages could lead to data breaches and loss of sensitive information.

Summary of Key Points

• E-mail is cost-effective: Most organizations already have e-mail systems in place, minimizing additional costs.
• Security features: E-mail can be encrypted and secured with various protective measures, making it suitable for confidential communication.
• Accessibility: E-mails can be accessed from multiple devices, allowing for flexible communication.
• Record keeping: E-mails provide a written record, which is important for accountability and future reference.

In conclusion, e-mail stands out as the most effective and secure method for transmitting confidential messages within an organization, making it the best choice among the given options.