Answer:
A. nucleus
Step-by-step explanation:
The cell nucleus is a structure present in eukaryotic cells, being, precisely, the main characteristic that differentiates them from prokaryotic cells, those that have the nucleoid genetic material. It is in the nucleus that this is the instructions for cells to differentiate and different types of tissues.
To facilitate understanding, an analogy is possible. Let's say the cells are industries, each department within the company is responsible for performing a specific function. In this case, these departments would be organelles, such as the Golgi complex, mitochondria, among others. The core, in this comparison, would be the management or administration sector. This is because it is exactly in the nuclear region that the genetic information that coordinates and commands cellular functions is present.