Q1 - It is probably within the reach

[chungyesol]

I thought (a) was an answer because it is mentioned in the beginning, and justification of research efforts just seems to support (a). On what grounds do we eliminate (a) and choose (d)?

[bbirdwell]

Here's what you are saying:

Research efforts are justified
THEREFORE
it is probably within the reach of technology to inhabit Mars

Does the first statement really support the second one?

Now consider this:
it is probably within in the reach of technology to inhabit Mars
THEREFORE
research efforts are justified

See the difference? This example is how the argument is logically structured. The fact technology is achievable is what leads to the idea that we should do research. Not the reverse.

[JinZ551]

And it is more likely that a conclusion will include some degree of judgement ("justified") or opinion, rather than pure fact.