“There is no reason to rule out the possibility of life on Uranus. We must, then, undertake the exploration of that planet".
The argument above assumes that
(A) life exists on Uranus.
(B) Uranus is the only other planet in the solar system capable of supporting life.
(C) Uranian life would be readily recognizable as life.
(D) the search for life is a sufficient motive for space exploration.
(E) no one has previously proposed the exploration of Uranus.
From the only stated premise “There is no reason to rule out the possibility of life in Uranus", the speaker comes to the conclusion that “we must undertake the exploration of that planet".
So, he assumes that the search for life is a sufficient reason for the exploration of other planets.
It is (D) which is closest to this statement, and is the answer.
The speaker only guesses that there may be life on Uranus. (A) raises the scope of his belief to the level of a certainty, and is wrong.
The speaker’s statement is confined to Uranus, and there is no reference in it to the possibility or impossibility or impossibility of life in any other planet in the solar system. So, (B) also extends the scope of his statement beyond what immediately follows from it, and is wrong.
Apart from speculating that there may be life on Uranus, the speaker does not speculate on how it will look like. So, (C) also extends the scope of the speaker’s statement, and is wrong. (If there is indeed life on Uranus, it may have thousands of different forms as it does on earth, and not just a single form!)
While it may true that none has previously explored Uranus, the statement that none has earlier proposed the exploration of Uranus also extends the scope of what the speaker says, and is not a necessary assumption for the conclusion.