Select the most appropriate option to substitute the underlined segment in the given sentence. If there is no need to as substitute it, select No substitution required.

A simple life of freedom and happiness is one of the more difficult thing to achieve.

A

No substitution required

B

one of the most difiicult things to achieve.

C

one of the most difiicult thing to achieve.

D

one of the more difiicult things to achieve.