Answer: b. It looked like she could knock it over by sneezing, but it was good enough for her to practice shooting.
Step-by-step explanation:
Something that is rickety is not very stable and could collapse at anytime.
In calling the item rickety and thinking that it could be blown over by sneezing meaning it could collapse at any time, the author is helping us understand that rickety means unstable much like the chair which is unstable and prone to collapse at a mere sneeze.