Answer : 3. "The airlines will send you the ticket by e-mail."
Explanation :

In Active Voice, a sentence emphasizes the subject, performing an action.
In Passive Voice, a sentence emphasizes the action or the object of the sentence.
The given sentence is in the passive voice and 'The ticket' is the subject and 'the airlines' is the object.
When we convert this sentence into active voice, the subject 'The ticket' of the passive voice becomes the object, the object 'the airlines' becomes the subject.
The passive format "will + be + V3 (sent)" should be changed into the active format "will + V1 (send)".
This is the active and passive voice rule for the future simple tense.