The given sentence is in the passive voice, as the subject 'Sarah's bag' receives an action 'was stolen'.
If this sentence is changed to active voice, then the subject is also changed, in order to retain the meaning of the sentence.
'Subject (Sarah's bag) + verb (was stolen) + adverbial (in the train)' changes to:
'New subject (Somebody) + verb (stole) + new object (Sarah's bag) + adverbial (in the train)'
The active form of 'was stolen' in the simple past tense is 'stole'.
This can be seen in option A, hence it's the right answer.
(The new subject 'somebody' is implied in the passive voice: Sarah's bag was stolen by somebody in the train.)
Option B: The verb form 'has stolen' expresses the present perfect tense whereas the sentence is in the past tense 'was stolen'.
Option C: 'Is stealing' expresses the present continuous tense, different from the past tense of the sentence. Hence the meaning has changed.
Option D: Similarly, 'had stolen' expresses the past perfect tense, different from the tense of the sentence.
Hence options B, C and D are incorrect.