Step 1: Understand the sentence.
The sentence "He is worried in his father's illness" suggests that the preposition "in" is incorrect in this context.
Step 2: Identify the mistake.
The verb "worried" is usually followed by "about," not "in," when referring to a person's concern over something. The correct phrase should be "worried about."
Step 3: Conclusion.
Thus, "in" is the incorrect word in this sentence.
Final Answer: \[ \boxed{\text{The incorrect word is 'in'.}} \]
Among scientists, the discovery of the double helix structure of DNA and the genetic code it incorporates is widely regarded to be one of the most significant scientific discovery of the twentieth century.
If the President knew that his allies would let him down so suddenly, he would have handled them with the greatest care.
Although the new policy aims to increase efficiency, reducing costs, and enhancing employee satisfaction, some employees feel that the changes are too abrupt and poorly communicated.