Final answer:
The correct past participle to complete the sentence is "fait".
Step-by-step explanation:
The correct past participle to complete the sentence should be fait.
In this sentence, the verb "faire" (to make/do) is used in the past tense. The past participle of "faire" is "fait".
Therefore, the sentence should be: "L'autre jour elles ont (fait) un très bon gâteau au chocolat."
Learn more about French verbs