Explanation:

- The International Energy Agency's World Energy Outlook 2018 focuses primarily on electricity.

- Option 1: Nuclear Energy - This involves generating energy through nuclear reactions. While important, this wasn't the specific focus of the 2018 outlook.

- Option 2: Solar Energy - This is harnessing energy from the sun. Solar is significant but not the main theme of the outlook in question.

- Option 3: Electricity - The 2018 report emphasizes the future role of electricity in global energy systems, addressing generation, distribution, and consumption.

- Option 4: Hydropower - This involves generating electricity from flowing water. It plays a part in the energy mix but wasn't the central theme.