Which of the following is NOT currently considered a source of usable energy?

• A. Solar radiation
• B. Wind
• C. Geothermal heat
• D. Dark matter

Answer: (D)

Verified step by step guidance

1. Step 1: Understand the concept of 'sources of usable energy' — these are natural phenomena or materials that can be harnessed to produce energy for human use, such as electricity or heat.
2. Step 2: Review each option given: Solar radiation, Wind, and Geothermal heat are all well-known renewable energy sources currently used to generate power.
3. Step 3: Recognize that 'Dark matter' is a theoretical form of matter that does not emit, absorb, or reflect light, and currently has no known method to harness it as an energy source.
4. Step 4: Conclude that since dark matter cannot be utilized with current technology or scientific understanding as a source of usable energy, it is the correct answer to the question.
5. Step 5: Summarize that the question tests knowledge of practical and theoretical energy sources, emphasizing the difference between established renewable energies and hypothetical concepts like dark matter.