IMPERCEPTIBLE :

Mean
Obvious
Humble
Subtle

The correct answer is D. Subtle.

Imperceptible means not able to be perceived or detected by the senses. Subtle means difficult to see, hear, or understand. Both words describe something that is not easily noticed. However, subtle has a positive connotation, while imperceptible has a negative connotation. Something that is subtle is often elegant or refined, while something that is imperceptible is often insignificant or unimportant.

The other options are incorrect because they do not accurately describe the word imperceptible. Mean means unpleasant or unkind. Obvious means easy to see or understand. Humble means modest or unassuming.