Answer
2-chloropropanoic acid
Work Step by Step
First, look at the product i.e. the ester. Now, the ester has two parts i.e. acid part and alcohol part. $CH_{3}-CH(Cl)-CO-$ is the acidic part present at the left side of the ester group. So, the acid is definitely $CH_{3}-CH(Cl)-CO-OH$.
Complete reaction is written below: