(B) doesn't do anything to the actual argument. It tells us that there are also other things that reduce the risk of heart disease, but that doesn't really affect the argument that people who take vitamin C are healthier than average. It's kind of like if someone says "drinking coffee makes you smarter", answer choice (B) says "exercise also makes you smarter". But that doesn't actually break the link between drinking coffee and making you smarter. Same thing here: (B) just introduces another thing that also reduces the risk of heart disease, but it doesn't attempt to break or weaken the causal link between consuming vitamin C and decreasing the risk of heart disease.

The conclusion here is that "people who take Vitamin C supplements tend to be healthier than average." What (D) does is say, essentially, that just this link between vitamin c and reducing heart disease isn't enough evidence for the conclusion, because these people who "regularly consume high doses of vitamin C", although maybe decreasing their risk of heart disease, are also reducing their resistance to common infectious diseases (which is a bad thing, you want to have high resistance to diseases!). So knowing that high doses of vitamin C can decrease the risk of heart disease, but it also reduces the strength of the immune system, we can no longer conclude that people who take vitamin c supplements tend to be healthier. What if their risk of heart disease is super super low, but they just keep getting the flu? Can we really say they're healthier than average? We just don't know! So that's why (D) weakens it.

I hope that helped!

Quick question about (C): doesn't incinerating much of the garbage contribute to net environmental damage? I get that I have to make that extra jump to say that it would be completely counterproductive to minimizing the overall amount of env. damage, but isn't it still kind of relevant?