Answer : 3. "Elaborate arrangements are being made for the party."
Explanation :

The given sentence is an example of Active Voice. As per the given question, we have to change it to Passive Voice.
The process of transformation is as follows:
The subject of the given sentence is 'They'.
The object of the given sentence is 'Elaborate'.
The subject will be put in place of the object and the object will be put in place of the subject.
'Are making' will be changed into 'are being made'.
However, the subject 'they' will not be used as 'by them' because it becomes the hidden subject here. This is so because here the action is the central focus and the actor can be any unknown/general entity.
Thus the correct sentence is: Elaborate arrangements are being made for the party.