Correct Answer: Indispensable

Solution : The correct choice is the second option.

The word indispensable is the most appropriate one-word substitution for the group of words because it means something that is absolutely necessary, essential, and cannot be done without. It conveys the idea that a particular thing or quality is so crucial that it is indispensable for a particular purpose.

The meanings of the other options are as follows:

• Insular means relating to an island or being narrow-minded and isolated, but it doesn't directly convey the idea of something being absolutely necessary.
• Isolated means being separated or in a remote location, but it doesn't specifically convey the idea of necessity.
• Intolerant means lacking the ability to accept or endure something, but it does not directly relate to the concept of something being absolutely necessary.