The correct answer is a) mitochondrion.
Mitochondria are often referred to as the powerhouses of the cell because they are responsible for converting the chemical energy stored in food into adenosine triphosphate (ATP), which cells use as a source of energy. This process, known as cellular respiration, takes place in the mitochondria and involves a series of metabolic reactions. While chloroplasts (option b) are involved in photosynthesis and produce energy in the form of glucose in plants, and ribosomes (option c) are responsible for protein synthesis, the mitochondria are specifically designed to create ATP from nutrients, making them essential for energy production in both plant and animal cells. The nucleus (option d) contains the cell’s genetic material but does not play a direct role in energy conversion.