It is probably within the reach of human technology to make the climat

Agreed with D.

A. It is probably technologically possible for humankind to alter the climate of Mars.

It is probably within the reach of human technology to make the climate of Mars inhabitable. It might be several centuries before people could live there, even with breathing apparatuses, but some of the world’s great temples and cathedrals took centuries to build. Research efforts now are justified if there is even a chance of making another planet inhabitable. Besides, the intellectual exercise of understanding how the Martian atmosphere might be changed could help in understanding atmospheric changes inadvertently triggered by human activity on Earth.

It is mentioned that human tech can make it inhabitable but it's nowhere mentioned that humans can alter the climate.
___________________________________

B. It would take several centuries to make Mars even marginally inhabitable.

It is probably within the reach of human technology to make the climate of Mars inhabitable. It might be several centuries before people could live there, even with breathing apparatuses, but some of the world’s great temples and cathedrals took centuries to build. Research efforts now are justified if there is even a chance of making another planet inhabitable. Besides, the intellectual exercise of understanding how the Martian atmosphere might be changed could help in understanding atmospheric changes inadvertently triggered by human activity on Earth.

It is mentioned that people MIGHT live there (even with breathing apparatuses) but it's nowhere mentioned that planet WILL become inhabitable.

____________________________________

C. Making Mars inhabitable is an effort comparable to building a great temple or cathedral. - No way !! This is nowhere close to the MAIN POINT.

____________________________________

D. Research efforts aimed at discovering how to change the climate of Mars are justified. CORRECT

It is probably within the reach of human technology to make the climate of Mars inhabitable. It might be several centuries before people could live there, even with breathing apparatuses, but some of the world’s great temples and cathedrals took centuries to build. Research efforts now are justified if there is even a chance of making another planet inhabitable. Besides, the intellectual exercise of understanding how the Martian atmosphere might be changed could help in understanding atmospheric changes inadvertently triggered by human activity on Earth.

So, Research efforts now are justified if there is even a chance of making another planet inhabitable AND It is probably within the reach of human technology to make the climate of Mars inhabitable. So here's our answer.

____________________________________

E. Efforts to change the climate of Mars could facilitate understanding of the Earth’s climate.

It is probably within the reach of human technology to make the climate of Mars inhabitable. It might be several centuries before people could live there, even with breathing apparatuses, but some of the world’s great temples and cathedrals took centuries to build. Research efforts now are justified if there is even a chance of making another planet inhabitable. Besides, the intellectual exercise of understanding how the Martian atmosphere might be changed could help in understanding atmospheric changes inadvertently triggered by human activity on Earth.

They DO say that - understanding how the Martian atmosphere might be changed could help in understanding atmospheric changes inadvertently triggered by human activity on Earth

There's a difference between - UNDERSTANDING how to change MARS's CLIMATE will help UNDERSTAND atmospheric changes on EARTH. and...
EFFORTS to change MARS's CLIMATE could help UNDERSTAND EARTH's CLIMATE

____________________________________

My only suggestion is - For Main Point, Draw the Conclusion questions - One needs to dig deeply and understand the CORE of the argument. Hope this helps .