Correct option is A
Option (a) is the correct idiom to fill in the blank.
"In a nutshell" means to describe something in a concise way.
Meanings of other options:
·
In a nutshell: In the fewest possible words.
·
In a stew: In a state of great anxiety or agitation.
·
In a huff: In an offended manner.
·
In a spin: In a state of agitation or confusion.