In the context of microbial growth curves, which line on a graph best represents the growth of a facultative anaerobe in the absence of $\mathrm{O}_2$?

Compare facultative anaerobes to other types of microorganisms: Obligate anaerobes cannot tolerate oxygen and may grow poorly or not at all in its presence. Facultative anaerobes, on the other hand, can grow in both conditions, but their growth rate is reduced without $\mathrm{O}_2$. This is why their growth in the absence of $\mathrm{O}_2$ is less than their growth in its presence but greater than that of obligate anaerobes in air.

Eliminate incorrect options: A line showing a rapid decline in cell number would represent cell death, which is not characteristic of facultative anaerobes in the absence of $\mathrm{O}_2$. A line showing maximal growth equal to that in the presence of $\mathrm{O}_2$ is incorrect because anaerobic metabolism is less efficient. A line showing no growth at all is also incorrect because facultative anaerobes can grow without $\mathrm{O}_2$.