Question

How much money did you give for the flowers?(voice change)
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Answer 1

$\huge\bold\red{ᴀɴՖᴡᴇʀ}$

The given sentence is in active voice, as the subject 'they' performs an action 'didn't give' against the object 'the money'.

To change to passive voice, the object takes the place of the subject and vice versa and the word 'by' is introduced as now the subject receives the action by the object. The verb form of the sentence is also changed from active form to passive.

'Subject (They) + verb (didn't give) + indirect object (me) + direct object (the money)' changes to:

'New subject (I) + verb (was not given) + direct object (the money)'

The new object 'by them' is implied.

This can be seen in option D, hence it's the right answer.

The pronoun 'they' is non specific, it does not refer to a particular person. Hence in passive voice it can be implied.