Correct option is D
The roughness grade number (N-grade) is a standardized classification used to specify surface roughness in manufacturing and machining processes. It is expressed in terms of arithmetical mean roughness (R_a), measured in micrometers (µm) or microinches.
For N4, the corresponding roughness value is 0.2 µm (Ra). This surface finish is considered very smooth and is typically achieved using grinding, honing, or superfinishing processes.
Roughness Grade Chart (ISO 1302 Standard):
