The given sentence is in the active voice, as the subject. 'They' performs an action 'asked' against the object 'some difficult questions'.
If this sentence is changed to passive voice, then 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 (asked) + indirect object (me) + direct object (some difficult questions) + adverbial (at the interview)' changes to:
'New subject (I) + verb (was asked) + direct object (some difficult questions) + by + new object (them)'
This can be seen in option C, hence it's the right answer.
Option
A: The words 'they asked me' are still in active voice.
Option B: Similarly, here too these words still remain the name. When the voice changes, the subject (they) and object (me) will interchange, and the word 'asked' will be changed to 'was asked by'. This cannot be seen here.
Option D: 'Am asked' expresses perfect tense whereas the given sentence is in simple past tense.
Hence options A, B and D are incorrect.