Photosynthesis is used as part of a premise: photosynthesis demonstrates plant dependence on animals, which in turn supports the main conclusion of mutual dependence. The conclusion is not about photosynthesis.

This can be properly inferred—(B) would be a correct MBT answer—but it isn’t stated, nor is it the argument’s focus. The conclusion is about the mutual dependence between plants in general and animals in general, not the dependence of a subset of animals on plants.

This may be an inference, but the main conclusion isn’t about the chemical composition of Earth or its atmosphere. Chemical composition in the atmosphere (carbon dioxide) is only referenced in a premise.

This isn’t stated in the argument. We are told that human activity prevents plants from depleting carbon dioxide, not oxygen. Even if (D) had gotten that right, it would be a premise rather than conclusion.

This is the conclusion that the rest of the argument sets out to support. The journalist is trying to convince the reader of the mutual dependence between plants and animals; every other part of the argument supports this idea.