There are two main purposes for plants. 1. Providing oxygen 2. serving as producers. As we know, plants take in the sunlight from the sun and make glucose with it. This glucose in its form is chemical energy that benefits both the plants, or the consumers that eat them, or some parts of the plant. It does not talk about oxygen or atmosphere except for nitrogen, which they actually get from the ground. Therefore, Converting Radiant energy into Chemical energy would be the right answer.
