An imperative sentence does not normally have a subject. It is used to express a command or request.
 The imperative sentence in the active and passive voice takes the following form:
V1 + Object.
Let + object + be + past participle.
Example:  Help him. (Active Voice) Let him be helped. (Passive Voice)
Following the active voice form, we finally get- Send this email immediately.

The correct answer is The police refused them entry in Delhi.
Key Points
The given sentence is in the Passive Voice.
As per the given question we have to change the sentence into Active Form.
The structure of the given sentence as follows: Subject+V2+Object.
(Active Voice) Subject (objective case)+was/were+V3+Object (subjective case).
(Passive Voice) 'Were refused' will be changed into 'refused'.
'They' will be changed into 'them'.

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.