Correct Answer: I was helped by my sister in packing my suitcase.

Solution : The most appropriate passive form is the third option.

Explanation :

Passive voice is the voice in which the object experiences an action rather than the person who performs the action. Hence, the object in the active sentence becomes the subject in the passive voice.

The structure of the passive voice is:

"Object  + was + past participle of the main verb + by + agent of action"

So, the correct sentence would be: "I was helped by my sister in packing my suitcase."