Answer:
B). 30 minutes.
Explanation:
As per the given situation, if the children's behavior prevents Kathy from repeating the same number she requires to dial the phone number within the next '30 minutes' as the encoded information(received information) last after that and this stage is followed by the process of storage that needs to be done with in stipulated time or else one will forget it. Encoded information reaches(or stored) in the long-term memory only when it is stored through rehearsal with in 20-30 minutes and one will not be able to retrieve/recall it in future. Thus, option B is the correct answer.