Answer: as busy as
Step-by-step explanation:
"The shop is not as busy as it was yesterday."
The objective here is to compare how busy the shop is today to how busy it was yesterday. To do so, the best comparison phrase grammatically speaking is, "as busy as".
The first option is incorrect due to redundancy stemming from the word "busier" being a comparison word on it's own without the further need for the words "as".
Second option is out because it is not a comparison phrase in this context. The best option is therefore the last one.