The sentence starts out fairly easily with a list of things that travelers to Mars would have to do:

A) endure low levels of gravity,
B) avoid large doses of radiation,
C) contend with the chemically reactive Martian soil, and
D) having to ward off contamination...

We can see that (D) does not fit in with the rest of the elements. In other words, the sentence is not parallel.

Answer choices (D) and (E) not only fail to fix the problem but also make it worse. They can be immediately eliminated. Answer choice (A), which merely repeats the sentence, can also be eliminated.

Which of the remaining two (B vs. C) best fixes the error that we identified in the original sentence? Answer choice (C) is the best answer. Answer choice (B) does not provide a verb that is in the same form as the rest of the verbs.