Answer: B. Some people think that NASA should focus on exploring the universe, but others believe that building a colony on the Moon is the most important goal, even though it will be difficult.
Step-by-step explanation:
A sentence that is structured in such a way as to emphasize the importance of one idea over another will usually have two ideas in it that are in conflict which means that one idea will have to be shown to be more important.
The sentence above shows two ideas being space exploration and colony foundation and how these ideas are in conflict. Either of them can be shown to be of more importance so this is the best option.