In energy economics, researchersMultiple select question.identify the best energy source to use in production.estimate the replacement rate needed to maintain the world's population.study how to find low-cost energy sources.explain the reasons for decreased fertility.

In energy economics, researchers:

1. Identify the best energy source to use in production: This involves analyzing different energy sources such as fossil fuels, nuclear energy, renewable energy etc., and determining which is the most efficient and cost-effective for production purposes.

2. Estimate the replacement rate needed to maintain the world's population: This is not typically a focus of energy economics. Replacement rate usually refers to the number of children each woman needs to have to keep the population size stable. It's more related to demographics and population studies.

3. Study how to find low-cost energy sources: Energy economists do indeed study this. They analyze the costs of different energy sources and try to find ways to reduce these costs. This could involve improving technology, finding more efficient methods of extraction, etc.

4. Explain the reasons for decreased fertility: This is also not typically a focus of energy economics. Decreased fertility can be influenced by many factors, including lifestyle, health, age, etc. This is usually studied in fields like public health, demography, or sociology.