Correct Answer: Tricky

Solution : The correct choice is the second option.

Explanation: The appropriate one-word substitute for the given group of words, describing something that requires exceptional skill or caution in performance, is "tricky". A tricky task involves difficulties or uncertainties.
The meanings of the other options are as follows:

• Blunt means dull or straightforward, lacking sharpness.
• Sustainable means capable of being maintained over the long term.
• Abstract means existing in thought or as an idea but not having a physical or concrete existence.

Therefore, the correct answer is tricky.