Answer:
Agar
Step-by-step explanation:
Cornstarch, flour and arrowroot are all starch based. Cornstarch is made from corn, flour is made from wheat or grains, and arrow root is made from different starches from tropical plants.
Agar is not starch based. It is made from red algae and is used as a gelatin substitute.
Therefore, agar is the option that is not a starch-based thickening agent.