Select the correct passive form of the given sentence.

They fired her because she was careless.

She was fired by them because she was careless.

The correct passive form of the given sentence "They fired her because she was careless" is She was fired by them because she was careless.

The active voice of the sentence is "They fired her because she was careless." This sentence is in the simple past tense, which is used to describe an action that happened in the past. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "she" and the main verb is "fire." The past participle of "fire" is "fired." So, the passive form of the sentence is "She was fired by them because she was careless."

The other options are incorrect because:

• She is being fired by them because she was careless." This sentence is in the present progressive tense, which is used to describe an action that is happening now but is not finished. However, the action of firing her happened in the past.
• She is fired by them because she was careless." This sentence is in the simple present tense, which is used to describe an action that happens regularly or now. However, the action of firing her happened in the past.
• She had been fired by them because she was careless." This sentence is in the past perfect tense, which is used to describe an action that happened in the past before another action happened. However, there is no other action mentioned in the sentence.