Scarcity exists a. when people consume beyond their needs. b. only in rich nations. c. in all countries of the world. d. only in poor nations.

Scarcity, an economic concept, refers to the fundamental economic problem of having limited resources to fulfill the unlimited wants and needs of people. Every nation in the world encounters this issue regardless of its wealth or level of development.

a. "When people consume beyond their needs": This statement partially describes the result of scarcity, as overconsumption can lead to resource depletion, but it does not adequately define scarcity itself. b. "Only in rich nations": Scarcity affects all nations, irrespective of their wealth. Rich nations have limited resources, and they face the challenge of allocating them efficiently as well. c. "In all countries of the world": This statement appears to accurately describe scarcity, since it recognizes that all countries must address the issue of limited resources to satisfy infinite wants and needs. d. "Only in poor nations": As mentioned earlier, scarcity affects all nations, not only poor ones. Hence, this statement is incorrect.

After evaluating each option, it becomes evident that the correct answer to the question is option c. "In all countries of the world". This option correctly describes the concept of scarcity and recognizes its ubiquitous nature that affects every nation, regardless of its wealth or level of development.