The correct passive form of the given sentence is:

☀ The President has been assassinated by the extremists.

So, the correct passive form is: "The President has been assassinated by the extremists."

The other options and their explanations:

☀ The President had been assassinated by the extremists.

• This option uses the past perfect tense, which indicates an action that was completed before another past action. It is grammatically correct but may not be the most suitable tense for the given context.

☀ The President is assassinated by the extremists.

• This option is in the present tense, which does not match the past tense used in the original sentence.

☀ The President was assassinated by the extremists.

• This option uses the simple past tense, which is a grammatically correct passive form but does not convey the same sense of ongoing relevance as the original sentence. The present perfect tense ("has been assassinated") is a better fit here to express that the action has an impact on the present situation.