What Do We Mean by the Diffraction Limit of a Telescope

Multiple Choice

What do we mean by the diffraction limit of a telescope?

A) It is the maximum size to which any telescope can be built.
B) It describes the farthest distance to which the telescope can see.
C) It describes the maximum exposure time for images captured with the telescope.
D) It is the best angular resolution the telescope could achieve with perfect optical quality and in the absence of atmospheric distortion.