Step 1: Identify the tense.
The given sentence is in the simple past tense (killed).
Step 2: Recall the rule for changing voice.
Simple past tense (active) changes to was/were + past participle (passive).
Step 3: Change subject and object.
Subject: the cat
Object: the mouse
In passive voice, the object becomes the subject.
Step 4: Form the passive sentence.
The mouse was killed by the cat.
Step 5: Conclusion.
Therefore, the correct passive form is option (B).