(a) A fox has been caught by the tiger.
(b) A fox was caught by the tiger.
(c) A fox is caught by the tiger.
(d) A fox had been caught by the tiger.
In the given sentence the subject ‘The tiger’ performs an action ‘caught’ against the object ‘a fox’.
If this sentence is changed to passive voice, then the subject is also changed, in order to retain the meaning of the sentence.
Subject (The tiger) + verb (caught) + object (a fox) changes to:
New subject (A fox) + verb (was caught) + by + new object (the tiger)
The passive form of caught in the simple past tense is ‘was caught’.
So the sentence becomes: A fox was caught by the tiger.
Hence option b is the answer.