Correct Answer: Agony

Solution : The first option is correct.

The word agony is a noun that specifically denotes intense physical or mental suffering or distress. It accurately captures the meaning of extreme pain or anguish, both physically and mentally.

The meanings of the other options are as follows:

• Autonomy: This word refers to self-governance or independence.
• Aviary: This word refers to a large enclosure for confining birds.
• Autocracy: This word refers to a system of government where one person has absolute power and is unrelated to the context of extreme suffering.

Therefore, the correct one-word substitute is agony.