Correct Answer: rationalise

Solution : The correct choice is the first option.

Rationalise means to provide logical or reasonable explanations for actions. In the context of the sentence, it suggests that these people are finding logical explanations for their sickening behaviour.

The meanings of other options are as follows:

• Essentialise means to reduce something to its essential qualities or characteristics.
• Scandalise means to shock, offend, or cause public outrage by one's actions or behaviour.
• Tantalise means to tease or torment with the promise of something desirable but out of reach.

Therefore, the complete sentence is: "Hard to believe that these people can justify and rationalise such sickening behaviour!"