When the plastic limit of a soil is greater than the liquid limit, then the plasticity index is reported as

Answer: Option C

Option A: negative
This option is incorrect. The plasticity index cannot be negative. It is either zero or non-plastic (NP) in such cases.

Option B: zero
This option is incorrect. Zero is used when the plastic limit is equal to the liquid limit, not when it is greater.

Option C: non-plastic (NP)
This is the correct option. When the plastic limit exceeds the liquid limit, the soil is considered non-plastic, and the plasticity index is reported as NP.

Option D: 1
This option is incorrect. The plasticity index is not arbitrarily set to 1 in any standard soil mechanics context.

In conclusion, the correct answer is Option C: non-plastic (NP), as this correctly represents the condition when the plastic limit is greater than the liquid limit in soil mechanics.