This is a really tricky question. If we read this passage as an objection to an antecedent claim we would break it down into saying - question about intelligent life is imprecise - cannot just decide to define intelligent life more precisely because we need to keep our definitions open to new possibilities. (if likely find int. life -> open to new possibilities).
The issue with C is that the passage isn't saying we can't define intelligent life, it's saying that defining intelligent life in a more precise manner won't help us to solve the question of whether intelligent life exists elsewhere in the universe. In fact, it will work against finding an answer to this question because it is likely we will find intelligent life in some new possibilities. Another issue is the difference between "precise" and 'adequately". The passage it is saying intelligent life cannot be very precisely defined, but it may nevertheless be adequately defined. (Say, for example, we defined intelligent life as "thinking and reacting to the environment". This might not be a precise definition because it can be hard to determine whether things think, but it could nevertheless be adequate).
D is correct because the passage is saying IF we defined intelligent life more precisely then we would be limiting ourselves because we will probably find intelligent life only if we leave the definitions only if we leave our definitions open to new unimagined possibilities. (likely find intelligent life -> leave open possibilities, but defining more precisely would close those possibilities, meaning it's unlikely and therefore counterproductive to finding intelligent life).
I hope this helps, feel free to follow up with further questions.
