If the sufficient and necessary conditions were swapped, this would prove our conclusion. As it stands, however, (A) only provides us with a necessary condition for a solution not being implemented.

We know that the consequences of iron-seeding on the ocean are not known. (D) tells us that we should wait to implement iron-seeding based on this information, which proves the conclusion.

Wrong trigger. We don’t know if the consequences of iron-seeing would be worse than global warming, only that we don’t know what those consequences would be.

Leads to the wrong conclusion. We don’t need to prove that iron-seeding should never be implemented, only that we should wait until we know the full effects.

Wrong trigger. We don’t know if iron-seeding might make global warming worse. We just don’t know what other side effects outside of global warming it might have.