Which word describes something that is difficult to detect or grasp by the mind?

The word "subtle" is appropriate because it captures the essence of something that is not immediately obvious or easily understood. In many contexts, subtlety refers to nuances or details that require careful observation or deep thought to appreciate fully. For example, subtle hints in conversation or subtle themes in literature can easily go unnoticed unless one pays close attention.

In contrast, the other words have meanings that suggest clarity or directness. For instance, "obvious" refers to something that is easily seen or understood, while "vivid" implies strong, clear, and powerful impressions or images. Similarly, "clear" denotes something that is easily perceived or understood, lacking any ambiguity. Therefore, these words do not capture the essence of something that is elusive or requires more cognitive effort to comprehend, making "subtle" the most fitting choice.