The correct answer is D. Living organisms.
Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary information encoded in genes, which can be transmitted to future generations. Another major theme is evolution, which explains the unity and diversity of life. Energy processing is also important to life as it provides the chemical energy to drive the biological processes that support life.
A is incorrect because plants are a type of living organism, not the study of living organisms.
B is incorrect because animals are a type of living organism, not the study of living organisms.
C is incorrect because bacteria are a type of living organism, not the study of living organisms.