The analogy is meant to show parallels between two sets of words. Fertile is to arable as vague is to clouded. If you understand either pair it can be used to decipher the meaning of the other pair you don't know.
A or d because there can't just be random commas
The little boy was implacable when his parents left him alone with the babysitter.
When the critic reviewed the painting of the rainforest, he could not hep but notice the artist's ardor for the outdoors.
In an effort to assuage angry customers, the store issues everyone a full refund.
Do not become so harried with work, you will forget to enjoy the important things in life like family.
As aloof creatures, most cats prefer to be alone.
I know that you don't need these any more but here are some that you could use in the future!
Answer:
I believe it to be Improve
Explanation:
I hope this helps, sorry if it doesn't!