Hi Jkjones!
You're right that this is a Justify question with conditional reasoning. And you correctly identified the conclusion as the first part of the first sentence. So let's break it down:
Premise 1: Lowers blood cholesterol levels

Lowers risk of hardening of arteries

Lowers risk of arterial blockage
Premise 2: Data correct

(Moderate exercise

Lowers blood cholesterol levels)
Note: For this premise, I put the second part in parentheses because I only get that conditional statement if the data are correct.
Conclusion: Moderate exercise

Lowers risk of arterial blockage
If this were a simpler Justify question, you might only have Premise 1 and the Conclusion. In that case, you would see that "Moderate exercise" is the new information in the conclusion and you would need to link it to the beginning of your chain ("Lowers blood cholesterol levels") so that you could follow it down to the conclusion. So, in the simpler version, a correct answer choice might be "Moderate exercise

Lowers blood cholesterol levels."
But this question switches things up by adding in Premise 2. Premise 2 almost gives you what you need to prove that conclusion by drawing a link between "Moderate exercise" and "Lowers blood cholesterol levels." But it only allows you to have that link IF the data are correct.
Answer choice (D), of course, tells us that the data are correct. And if the data are correct, that means we have the "Moderate exercise

Lowers blood cholesterol levels" link which we can chain with Premise 1 to arrive at the conclusion.
Answer choice (C) is something that must be true based on the information we have above. It is essentially just Premise 1, if you eliminate the middle condition ("Lowers blood cholesterol levels

Lowers risk of arterial blockage"). But it doesn't add anything to our argument and therefore can't prove our conclusion about "Moderate exercise."
Hope this helps!
Best,
Kelsey