Means camera obscura.

a. dark camera

b. obscure room

c. dark room

It depends on what type of third separates the three notes.for example: if you have two mayor thirds is an "aumented chord"if you have two minor thirds is an "diminued chord"if you have one mayor first and then one minor is a "mayor chord"and inverted is a "minor chord"

Im not sure but i know it is not b