Question

change into voice- we elected balu captain

Answer 1

Voice change,

The following sentence have a compliment,

So you should follow these rules

The sentences which have a transitive verb normally can be converted to passive with either of the objects.But I personally recommend use the personal object (indirect object) for the subject in passive .

Because some sentences contains transitive verb but sometimes they also contain a compliment.

In that case you can't really use the direct object in your case it's captain.

Explaining the rules required let's forward
I think now you can do it by yourself
But you had cost points to this question

So the passive form is,

Balu was elected captain by us
Incorrect : Captain was elected balu by us (because captain is used as a compliment)

That's it
Hope it helped (･ิω･ิ)

Answer 2

Answer:

Balu caption was elected by us