Well to understand this better, it would be beneficial to first analyse what are Autotrophs
Autotroph is an organism that can produce its own food by using light , water and carbon dioxide. And Since autotrophs produce their own food so they are sometimes also called producers.
Now since these characteristics fall in all plants categories thus it is very much true that all plants are Autotrophs because they make their own food by the process of photosynthesis in the presence of sunlight and water. They don't need anyone to make their food. They take carbon dioxide from air, sunlight from the atmosphere and water from the soil and make glucose and oxygen.