Answer: Option B: They don't understand how redshifts affects stars.
There are millions and billions of stars in our universe. Scientists study stars using telescopes. But some stars are so far, that even the best powerful telescopes are not able to detect their light. Also, the interstellar distance is filled with clouds of dust and gas which makes some part of the universe opaque. It becomes difficult to study stars using earth based telescopes due to lot of light pollution. Astronomers understand how redshifts affects stars and hence, are able use this to study stars via their spectrum. Hence, astronomers don't understand how redshifts affects stars is a wrong reason of why they face difficulty in looking at distant stars.