How will the right answer fit in terms of support and conclusion?

Only the right answer will add support for the conclusion. You know a “principle” is pretty much always support.

Highlight the main conclusion in the passage, if there is one:

the discovery of powerful cures for certain high-incidence forms of cancer would probably trigger an increase in behaviors…that are known to increase the risks of such cancers.

Journalist: [SUPPORT]. Thus, [CONCLUSION].

Map the wording of the answers to the wording of the passage:

(A) …they will take only the measure that they believe will most reduce the risk.

That doesn’t map to the passage, which doesn’t mention what “they believe will most reduce the risk”.

(B) The development of ways to protect people…often makes people less careful…

Yup, that’s exactly what the passage says. The first part maps to “the discovery of powerful cures” and the second part maps to “an increase in behaviors…that are known to increase the risks”.

(C) …do not take special care to avoid behaviors that they believe will not harm them at all.

Ohh, that might’ve been okay until the end. The passage only says they’re more likely to do risky stuff if there’s a “powerful cure” available, but that doesn’t mean they believe cancer is totally harmless.

(D) …performing activities of unknown risk.

That wording doesn’t map to anything in the passage. The argument is about “discovery of powerful cures”, in other words it’s about what happens when we know the risk has gone down.

(E) …is frequently outweighed by various desires.

The passage never mentions “desires” or weighs them against the risk of “serious harm”.

(B) is the correct answer.