Pluto is pretty far away from our planet, the Earth. It'd take a lot of years for the research and many years for the return.
So the answer is D.
For all other choices.
A is pretty wrong. As you should know that Pluto is VERY far from the Sun.
B is an assumption but not really that much logical.
For C, we know where Pluto is.
D is the most logical answer for the question as it is too far away for the research. After all It'd take more than approximately 10 years.
