We must explore . . . . . . . . sources of energy as our supply of fossil fuel has been . . . . . . . .

The other options are incorrect for the following reasons:

• Guaranteed means “certain to happen or be the case.” This is not the case for our supply of fossil fuels, as it is finite and will eventually run out.
• Natural means “existing in or produced by nature.” Fossil fuels are natural resources, but they are not renewable.
• Sufficient means “enough or as much as is needed.” Our supply of fossil fuels is not sufficient to meet our current and future energy needs.
• Increased means “made or become larger or greater.” Our supply of fossil fuels is not increasing, but rather decreasing.
• Innovative means “characterized by the introduction of new ideas or methods.” While there is some innovation in the development of alternative energy sources, this is not the main reason why we need to explore them. The main reason is that our supply of fossil fuels is running out.