Right answer:
b. comiendo
This is the correct answer because it is the more logical option. This sentence is given in the past progressive that we use when talking about something that was happening now at a certain moment in the past. This is formed by the verb estar in the past plus a present participle following the formula:
estar (in the past) + present participle
The present participle is formed using the ending ndo that matches the ing in English. On the other hand, estaban is the conjugation of the verb estar for the third person plural in the imperfect, which is a past tense. Finally:
Mis hijas estaban comiendo una quesadilla a las diez