The correct answer is D. indispensable.
Manpower is the indispensable means of converting other resources to mankind’s use and benefit. Without manpower, it would be impossible to extract and process raw materials, build and operate machines, or create and maintain infrastructure. Manpower is also essential for providing essential services such as healthcare, education, and security.
The other options are not as accurate. A. insuperable means “impossible to overcome”. While manpower can be a powerful force, it is not impossible to overcome. B. inimitable means “not able to be copied”. While manpower is unique in its ability to think and act, it is not impossible to replicate. C. indivisible means “not able to be divided”. While manpower is a collective force, it can be divided into smaller groups. E. inequitable means “unfair”. While manpower can be used for good or evil, it is not inherently unfair.