OPAQUE

Opaque means something that cannot be seen through; light does not pass through it. Figuratively, it also means difficult to understand, obscure, or unclear.

Transparent means something that can be seen through completely; light passes through it freely. Figuratively, it means open, obvious, and easy to understand, with nothing hidden.

Why not clear: Though clear seems like a close contender, it is not the right choice because clear can mean a broader number of things; it can mean something that can be seen through clearly, but it can also mean something free from cloudiness, darkness or dirt. 

Because, we have a more precise and exact option present we choose transparent.