The condition for observing diffraction from a single slit is that the light wavefront incident on the slit should be

Plane

The correct answer is Option (3) → Plane

Diffraction occur when light passes through a slit and spreads out, for this to happen, the wavefront of the incident on the slit should be plane, meaning the light should arrive as parallel rays. This is the typical situation where diffraction pattern can be clearly observed.