The correct option is D permanent
The word 'transient' refers to something lasting only for a short time.
Option A, permanent refers to remaining unchanged indefinitely.
Option B, sinful refers to something immoral and corrupt.
Option C, unnatural refers to something contrary to what is seen as normal, conventional or acceptable.
Option D, joyful refers to feeling, expressing or causing great pleasure and happiness.
Thus, according to the meaning, the word 'permanent' is the opposite of the word 'transient'.