The correct option is C The third paragraph describes a specific case of a phenomenon that is described generally in the second paragraph.
The third passage starts by ‘An important example of this phenomenon’. The ‘this’ implies that something was mentioned in the previous paragraph that was carried to the third paragraph. In the second paragraph it is mentioned the case of ‘concentrations of most mRNA’s correlate best, not with their synthesis rate, but rather with the equally variable rates’. This is given with an example phenomenon in third paragraph. Hence option(C) is the answer.