Activities that increase the skills and knowledge of workers.

• A. Invest
• B. Keystone
• C. Invoice
• D. Investment in Human Resources

Answer: (D)

This item is about activities that boost workers' skills and knowledge, essentially investing in people. An investment in human resources explicitly refers to programs and resources aimed at developing employees’ abilities—training, education, mentoring, leadership development, and similar learning initiatives. These activities directly raise capabilities, productivity, and value to the organization.

The other options are too vague or unrelated. "Invest" is a general term that could apply to many things and doesn’t specify people. "Keystone" refers to a central element or architectural piece, not employee development. "Invoice" is a billing document, unrelated to increasing skills.