Дополните предложение нужной формой глагола. Can you tell me when Mrs. Ellis... back?

will be back is the correct answer.

Here's why:

The sentence "Can you tell me when Mrs. Ellis... back?" requires a future tense to indicate when Mrs. Ellis will return. "Will be back" is the correct future tense form.

Thus, the completed sentence is: "Can you tell me when Mrs. Ellis will be back?"