Question

his reply surprised me( passive voice)

Answer 1

It cannot be changed because it is intransitive verb

Answer 2

here is your answer

his reply surprised me
-- I was surprised by his reply.

Explanation 》||
the object of the active voice i.e me will change to the subject of passive voice that is 'I'.

the active voice is in past tense so we will use was or were and 3rd form in pasdive voice.

then add 'by' after that the subject of the active voice will become object of passive voice.



hope it helps you