Answer:
B. In this way of thinking, the "background" acted more like a flexible mat, or a stretchy knit blanket.
Step-by-step explanation:
Essentially, we want to pick the choice that will help readers picture the scientific ideas more clearly and easily by linking it back to something that regular people (non-scientists) are familiar with.
Look at the answer choices:
A: Mercury's orbit-up close to the Sun--has a blip in it that Newton's laws can't explain.
There really isn't anything here that can help readers visualise this or that relates to their own experience - who has ever experienced Mercury's orbit?
B: In this way of thinking, the "background" acted more like a flexible mat, or a stretchy knit blanket.
Ah, now here's something many people are familiar with: a flexible mat or a stretchy knit blanket. This definitely helps people understand what kind of image to picture when reading about the black holes. So, B is correct.
C: Scientist Karl Schwarzschild began to think about what would happen if a place in space were extremely distorted.
Nothing in here suggests something regular readers have experienced, so this isn't right.
D: All of this is powerful evidence that Einstein was on to something.
While we all know who Einstein is, we must remember that we've never met Einstein, nor have we ever really experienced something related to him on a daily basis. So, D is incorrect.
The correct answer is thus B.