Final answer:
DNA contains the code that guides all cell activities by providing the necessary instructions to build proteins, making it the correct answer to the question. It is the genetic material that serves as a blueprint for protein synthesis, crucial for the function and structure of cells.
Step-by-step explanation:
The substance that contains the code which guides all cell activities is DNA. DNA serves as the blueprint for cell structure and physiology and contains the genetic information required to build proteins. Proteins, including vital enzymes, carry out nearly all cellular functions. The sequence of amino acids in proteins is determined by the nucleotide sequence in DNA, with each gene coding for a specific protein. This process, from DNA to protein synthesis, is a fundamental aspect of the cell's operation. While RNA plays a crucial role in this process as the messenger molecule that helps to create proteins based on DNA's instructions, DNA itself is the genetic material that provides the information for synthesis, making it the answer to the question.