Irrelevant. We were already told that the carbon dioxide will be pumped into cold water.

The oceanographer isn’t saying that the carbon dioxide will never escape, only that it will take centuries. If it is released after that point, the argument is unharmed. Also, if warm water releases carbon dioxide other than the carbon dioxide that was pumped into cold water, while the cold-water carbon dioxide remains trapped, the argument is also unharmed.

This must be true, and it matches our prediction. (C) is making the mixing of cold and warm water necessary for carbon dioxide release. If this is not true, and carbon dioxide can escape whether the water mixes or not, the argument has no support.

It doesn’t matter how the trapping happens. Additionally, (D) doesn’t specify in what way density is related to trapping: Is higher density or lower density preferred? We don’t know, and we can’t assume.

The oceanographer is not arguing that trapping the carbon dioxide for centuries is required for pumping it into the ocean. She is only arguing that it is a reason to do it.