The correct answer is B.
It is reasonable to assume that the graph flattens due to limits of space and available nutrients. These are apparently the only independent variables in the experiment. Thus, either or both factors must be limiting the growth of microorganisms.
Choice (A) is not correct because neither the graph nor the experimental setups describe a time-growth relationship. In other words, there is no data upon which to base the conclusion represented by choice (A).
Choice (C) is incorrect. Density can be thought of as "thickness." Increased growth produces a thicker medium that inhibits the passage of light. Hence, optical density increases with cell growth. They are
directly proportional, not
inversely proportional.
Choice (D) isn't supported by any of the data. Here, light is simply being used to measure the growth of microorganisms.
