Which one of the following expressions is used for comparing producti…

The correct answer is Option (2) → gm⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹

A rate of production has to show three things:

• the amount: weight in grams (g) or energy in kilocalories (kcal);

• per unit area: per square metre, m⁻²;

• per unit time: per year, yr⁻¹.

Option (1) kg⁻¹ yr⁻¹ or (kcal m⁻¹) yr⁻¹ — Incorrect. Productivity is measured per square metre of area, not per kilogram or per metre.

Option (2) gm⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹ — Correct. Weight or energy, per square metre, per year. With the same unit for every ecosystem, their productivity can be compared.

Option (3) gm⁻² yr⁻² or (kcal m⁻²) yr⁻² — Incorrect. The area unit is right, but yr⁻² is not a rate per year.

Option (4) gm⁻² yr⁻¹ or (kcal m⁻¹) yr⁻² — Incorrect. The first unit is right, but the second is wrong twice: area must be m⁻², not m⁻¹, and time must be yr⁻¹, not yr⁻².

NCERT Extract:
Productivity – “A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g m^–2) or energy (kcal m^–2). The rate of biomass production is called productivity. It is expressed in terms of gm^–2 yr^–1 or (kcal m^–2) yr^–1 to compare the productivity of different ecosystems.”