The correct answer is option 'C': Species evenness relative abundance.

Explanation:
Species evenness refers to the distribution of individuals among different species within a community or ecosystem. It measures how evenly the individuals of different species are distributed and whether the numbers are equal or skewed.

Relative abundance, on the other hand, refers to the proportion or percentage of individuals of a particular species compared to the total number of individuals in the community or ecosystem. It quantifies the number of individuals of each species.

When we talk about the extent to which numbers of different species are equal or skewed, we are considering both the species evenness and the relative abundance. This combination gives us a more comprehensive understanding of the diversity and composition of a community or ecosystem.

Here is a breakdown of the options:

a) Relative abundance only: This option focuses solely on the proportion of individuals of different species without considering the evenness of their distribution.

b) Species evenness only: This option considers only the evenness or equality in the distribution of individuals among different species, without taking into account the relative abundance.

c) Species evenness relative abundance: This is the correct option as it considers both the evenness of species distribution and the relative abundance of individuals.

d) Species richness species evenness: This option combines species richness, which refers to the total number of different species in a community or ecosystem, with species evenness. However, it does not consider the relative abundance of individuals.

In summary, option 'C' is the most appropriate answer as it takes into account both the species evenness and the relative abundance to determine the extent to which numbers of different species are equal or skewed in a community or ecosystem.