Question

intangible (adj.)
feeling or showing little or no emotion
incapable of being perceived by the senses
obligatory; mandatory; subject to a command or order.
full of melancholy; vague or regretful longing

Explanation:

The word "intangible" describes something that cannot be touched, seen, or detected by the physical senses, which matches the second option. The other options define unemotional, mandatory, and melancholic/regretful states respectively, which are unrelated to the meaning of "intangible".

Answer:

incapable of being perceived by the senses