The main idea of the paragraph is that food serves as a source of energy, replaces worn-out cells, and adds fluids to the body. This is supported by the statement that "Makanan berfungsi sebagai sumber energi, mengganti sel-sel yang sudah usang, dan menambah cairan tubuh" (Food serves as a source of energy, replaces worn-out cells, and adds fluids to the body). Therefore, option A accurately summarizes the main idea of the paragraph.