Answer: B. To generate energy through cellular respiration
Explanation: The primary purpose of the mitochondria is to perform cellular respiration. During the process of cellular respiration, glucose is broken down and oxidized in order to create ATP, a form of cellular energy used by cells in order to carry out its functions and survive. Let's go over each of the other answer choices to determine why they're wrong.
Choice A is describing the role of ribosomes. Ribosomes are plentiful organelles found in the cell that are responsible for translating mRNA molecules into their respective proteins.
Choice C is describing the role of the nucleus. The nucleus is a large spherical organelle that houses the DNA of the cell. Thus, the nucleus' purpose is to house DNA and regulate the activity of the cell.
Choice D is describing the function of the cell membrane, a semi-permeable membrane that separates the contents of the cell from the outside environment. In addition, it is also responsible for controlling what molecules can and can't enter the cell.
As we can see, only choice B describes the function of the mitochondria. All of the other choices describe the functions of other organelles.