Passive to active interrogatives

Solution 1:

Has anyone told them to be ready?

is the translation into active voice.

Solution 2:

The active voice statement would be Someone told them to be ready. The question simply follows the rule that you insert an auxiliary verb, do or have and invert the subject and auxiliary. We don't know who it is who was or is supposed to tell them to be ready, so I inserted someone.

(A) Has someone told them to be ready?

Another possibility is this:

(B) Has anyone told them to be ready?

The meanings of these two sentences are slightly different, but because you seem to have been given a grammatically incorrect sentence to change from passive to active, you might also be given an answer that a native speaker of British or North American English wouldn't consider normal. I'd say that (A) should be the book's answer, but it might be (B).

Solution 3:

Technically speaking, the answers offered so far aren't the only possibilities.
One might also suggest

"Have you/she/we/they/folks told them to be ready?"

According to common rules, those forms are "supposed" to be translated into the passive like this:

"Have they been told to be ready by you/her/us/them?"

But these questions are artificial (i.e., not used) outside of contexts where the specific person(s) that are doing the telling are being queried--in other words, outside of the context in which the questioner is trying to figure out who it is that might or might not have told them to be ready. This contrasts the likeliest context for asking "Have they been told to be ready?", in which the concern is whether or not they have been told to be ready, and maybe whether or not you told them to (not just anyone).

From the standpoint of pragmatics (in the linguistic sense), because the earlier concern is not the likely focus of the question in "Have they been told to be ready?", it need not be the focus in any translation of the question to the active voice.

And so I submit

"Have you/she/we/they/folks told them to be ready?"

as alternative answers to the question.