Questions: What did the modern synthesis add to the theories of Darwin and Wallace? Mechanisms of inheritance of traits. The concept of natural selection. A modern understanding of cellular respiration. A framework for understanding limited resources.

The answer is the first one: mechanisms of inheritance of traits.

Explanation for each option:

• Mechanisms of inheritance of traits: This is correct. The modern synthesis, which emerged in the early to mid-20th century, integrated Darwin and Wallace's theory of natural selection with Mendelian genetics. It provided a genetic basis for the inheritance of traits, explaining how traits are passed from one generation to the next through genes.

• The concept of natural selection: This is incorrect. The concept of natural selection was already a central part of the theories proposed by Darwin and Wallace in the 19th century. The modern synthesis did not add this concept but rather expanded upon it by incorporating genetic principles.

• A modern understanding of cellular respiration: This is incorrect. The modern synthesis did not focus on cellular respiration. It was primarily concerned with the integration of evolutionary biology and genetics.

• A framework for understanding limited resources: This is incorrect. The idea of limited resources and its role in natural selection was already part of Darwin's original theory. The modern synthesis did not specifically add to this concept.

In summary, the modern synthesis added the mechanisms of inheritance of traits to the theories of Darwin and Wallace, providing a genetic framework for understanding evolution.