Isotopes are atoms of the same element that differ in the number of neutrons they contain. They keep the same number of protons, so the element’s identity (its atomic number) is unchanged, but the mass changes because neutrons add mass without changing charge. For example, carbon-12 and carbon-14 have six protons, but different numbers of neutrons, giving them different mass numbers. Because the protons (and electrons) largely determine chemical behavior, isotopes of the same element behave similarly chemically, though their nuclei differ in stability and some physical properties. If you change the number of protons, you’ve created a different element entirely, not an isotope. If you change the number or arrangement of electrons, you get an ion, which is charged but not defined by a different neutron count. And a molecule is simply a bonded collection of atoms, not a description of a single atom’s neutron content.

Energy efficiency is the ratio of useful work or heat output to the total energy input. It shows how effectively energy is being used—when this ratio is higher, less energy is wasted as heat or other losses. In energy planning, boosting efficiency means you can achieve the same service with less energy, lowering operating costs, reducing energy demand, and cutting environmental impact. For example, better insulation, more efficient equipment, and heat recovery in processes let you get the same result with less energy. The other ideas misrepresent what efficiency means: simply increasing input for more output isn’t efficiency, and imagining no losses or a focus on maximum output ignores how much energy is wasted.

Hydrocarbons originate from organic-rich sedimentary rocks that have been buried enough to heat up and mature the contained organic matter. The best source rocks are fine-grained, organic-rich sediments—think shale or mudstone—because their small particles help preserve organic material and create the right conditions for low decay and preservation of kerogen. When these rocks are buried to sufficient depths, the heat and pressure transform the organic matter into hydrocarbons, producing crude oil and natural gas. Saying the rock is fine-grained and organic-rich and buried at tens to hundreds of meters (depending on geothermal gradient) captures the essential idea of a source rock. The other options don’t fit: coarse sandstone with little organic content lacks enough organic material to generate significant hydrocarbons; an igneous rock with high silica contains little to no preserved organic matter; metamorphic rock formed under heat typically has altered or SAMPLEexpelled any original organics and isn’t a common source rock for oil and gas.

Renewable energy sources are those that are naturally replenished on human timescales. Hydropower taps into flowing water, which is constantly renewed by the water cycle—rainfall, rivers, and streams keep supplying water for power generation, so it can be used repeatedly without running out soon. In contrast, coal and oil come from ancient organic matter formed over millions of years and are depleted much faster than they form, making them nonrenewable. Solar energy is also renewable, captured from sunlight, but hydropower is a clear example among these options.

Understanding how thermal efficiency is defined helps explain these numbers: it’s the fraction of the fuel’s energy that actually becomes electricity. Conventional coal-fired plants run a simple steam cycle, so they typically convert about one-third to two-fifths of the input energy into electricity, roughly 33–40%. Natural gas combined-cycle plants pair a gas turbine with a steam turbine, using the waste heat from the gas turbine to power an additional steam cycle, which raises overall efficiency to about half to sixty percent in common practice. So the statement that coal-fired plants are around 33–40% efficient and natural gas combined-cycle plants are around 50–60% efficient matches these real-world ranges. The other options propose much higher or lower values than what is typical for these technologies.