Some people . . . . . . . . themselves into believing that they are indispensable to the organisation they work for.

The correct answer is delude.

To delude is to deceive or mislead someone, especially by making false promises or creating false impressions. In this case, the people are deceiving themselves into believing that they are indispensable to the organization they work for.

The other options are not as accurate.

• Keep: This means to maintain or retain something. It does not make sense in the context of the sentence.
• Fool: This means to deceive someone, especially by deliberate trickery. It is possible that the people are fooling themselves, but this is not the most likely interpretation of the sentence.
• Force: This means to compel someone to do something against their will. It does not make sense in the context of the sentence.
• Denigrate: This means to speak or write about someone or something in a very negative way. It is possible that the people are denigrating themselves, but this is not the most likely interpretation of the sentence.