Final answer:
The genetic material in cells, particularly DNA, is central to directing cellular activities by coding for the production of proteins and transferring genetic information to offspring.
Step-by-step explanation:
The sentence that BEST emphasizes what the author means by "directing cellular activities" is: DNA is the genetic material responsible for inheritance and also serves as the information to direct and regulate the construction of the proteins necessary for the cell to perform all of its functions. The genetic material, which includes both DNA and RNA, is pivotal for not only passing genetic information to offspring but also for the daily operational functions of the cell, including the production of proteins essential for cell structure and function. DNA does this by organizing into genes which code the sequences of messenger RNA (mRNA), which, in turn, specifies the sequence of proteins that determine a cell's phenotype under various environmental conditions.