Which of the following represent special types of materials that may contain classified information?

Special types of materials that may contain classified information encompass a variety of documents, including research reports, technical specifications, and meeting notes.

Research reports often provide detailed findings that might include sensitive data or proprietary methodologies, thus making them subject to classification to protect national security interests or intellectual property.

Technical specifications typically delineate standards, designs, or configurations for systems and equipment which can reveal critical capabilities or vulnerabilities if disclosed. This makes it essential to classify these documents to ensure that potentially harmful information does not fall into the wrong hands.

Meeting notes may seem innocuous, but they can include decisions and discussions regarding classified projects or sensitive information that are also subject to protective measures. The nuances and insights shared during meetings can carry implications that necessitate classification.

Given that all these types of documents can contain sensitive information relevant to national security or operational integrity, the most accurate answer is that they all may fall under the purview of classified materials.