A few weeks after marion had her phone number changed, she wrongly gave out her former phone number. the most likely explanation for remembering the old number rather than her current number is

The correct answer is proactive interference.

Proactive interference refers to the phenomenon wherein, old memories get in the way of or interfere with retaining and retrieving new memories. In this instance, Marion's old memory of her former phone number is interfering with her ability to retrieve the memory of her (newer) current phone number.