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The first step to answering this question is to understand what each of these terms mean:\n\n- Fundamental niche: This refers to the full range of environmental conditions under which a species can survive and reproduce.\n\n- Realized niche: This is the portion of the fundamental niche that is actually utilized by the species under certain conditions.\n\n- Character displacement: It happens when similar species that occupy the same environment adapt to minimize competition for available resources.\n\n- Resource partitioning: It involves the dividing of resources amongst species so as to decrease competition.\n\n- Relative abundance: It is a component of biodiversity and refers to the evenness of distribution of individuals among species in a community.

Once you have understood the meanings of all the terms, the next step is to apply them to the question and find which one fits the description.

The phrase 'the range of resources a population can possibly use' best defines the fundamental niche of a species. Thus, the correct answer is Option (a) - its fundamental niche.