Which one of the following values given in terms of redox-potential of soils (mv) represents a highly reduced condition of the soil

Answer: Option A

-250 is the one among the following values given in terms of redox-potential of soils (mv) representing a highly reduced condition of the soil.
Redox potential indicates how oxidized or reduced the soil is as well as the electron availability in the soils. It is usually determined by measuring the potential difference between an inert electrode installed in the soil and a reference Ag/AgCl or calomel electrode at the soil surface.