A tillage system when 15-30% soil surface remains under crop residues cover after planting is called:

Correct option is C

A tillage system where 15-30% of the soil surface remains covered with crop residues after planting is called (c) Reduced tillage. 
· Zero Tillage (No-Till):
This system involves planting crops directly into undisturbed soil, with no prior tillage, and leaving all crop residues on the surface. 
· Conservation Tillage:
While reduced tillage is a type of conservation tillage, the term "conservation tillage" generally refers to any tillage system that leaves at least 30% of the soil surface covered with crop residue. 
· Intensive Tillage:
This involves frequent and thorough soil disturbance, often leaving little to no residue cover.