(A) It is a claim for which no justification is provided but that is required in order to establish the argument's main conclusion.
(B) It is a claim for which no justification is provided and that, if true, ensures the truth of the argument's main conclusion.
The above two choices are easy eliminations since the argument does give some justification. The justification is that Mars did not get as severely bombarded as Earth did and hence the surface would not have been sterilised, allowing microbes to grow.
(C) It is a claim for which some justification is provided and that is required in order to establish the argument's main conclusion.
(D) It is a claim for which justification is provided and that, if true, establishes the truth of the argument's main conclusion.
Better to look at C and D together. The only difference in these two choices is the extent of justification provided. Clearly, the fact that asteroids did not severely bombard is only part of the justification for microbes to exist. Climate, etc are also factors for which justification has not been given. Hence, C looks a better choice. Eliminate D.
(E) It is a claim that provides some support for the argument's conclusion but that neither ensures the truth of that conclusion nor is required in order to establish that conclusion. -- First part of the statement is correct, but this claim does ensure that the main conclusion (mars brought microbial life to earth) survives. Eliminate E.