Answer
(a)
Work Step by Step
Denitrification is the microbial conversion of nitrate ($NO_3^⁻$) into nitrogen gas ($N_2$) or nitrous oxide ($N_2O$), which occurs under anaerobic (low-oxygen) conditions.
Freshly tilled topsoil is well-aerated, so oxygen is abundant, which inhibits denitrifying bacteria.
The answer is (a).
