If necessary to design a new experiment, which of the following best explains why researchers could use measurements of intracellular lactate levels (ill) in cancer cells to assess efficacy of cancer drugs?

As we know that lactate is an essential part of glycolysis which formed by the process of fermentation. According to question cancerous cells depends on fermentation to get energy.  

By inhibiting fermentation using drug results in reduction of ILL which indicate decline on cancerous cell metabolism.

Furthermore, using cancer drug, we will see low production of lactate which means drug efficiently inhibited the energy supply to cancerous cell by inhibiting fermentation.

currents is the correct answer.