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 active voice and 'We' is the subject and 'Liza' is the object.
When we convert this sentence into passive voice, the subject 'We' of the active voice becomes the object 'us', the object 'Liza' becomes the subject.
The passive format "is + V3 (regarded)" should be used. This is the active and passive voice rule for the present simple tense.

If a verb requires an object after it, it is a transitive verb.
If a verb doesn't require an object after it, it is an intransitive verb.
For example:
The girls carry water to their village. (transitive verb) A light was shining. (intransitive verb)
There are certain verbs that are used in a particular pattern given below:
Subject + verb + noun/pronoun + present participle(-ing form).
Some of the verbs are given below:
Hate/love, stop/start, finish, enjoy, prefer, keep, rest, worth, propose, regret, etc.
Example: She hates going out in the night.
According to the rule and the example given above, 'has started bringing' will be used instead of 'has started bring' in the 2nd part of the sentence.

The past simple tense is changed into the passive voice form.

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.