6696. Decay is an immutable factor of human life.

✅ The correct answer is option C.
Important : of great significance or value.

Unique : being the only one of its kind; unlike anything else.

Unchangeable : not liable to variation or able to be altered.

Awful : very bad or unpleasant.

Immutable : unchanging over time or unable to be changed.

Synonym of Immutable is Unchangeable