Question

They laughed at the poor man. (Change the voice.)

i. The poor man was laughed at by them.

ii. The poor man laughed at them.

iii. The poor man is being laughed at by them.

iv. The poor man laughed at by them.​

Answer 1

ii. The poor man laughed at them (passive voice)

it is not mandatory that whenever we have to change it to passive voice we need to use "by"