The rate of biomass production is called productivity and can be expr…

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

Productivity is a rate: the amount produced on one unit of area in one unit of time. So its unit must be ‘per square metre’ (m⁻²) and ‘per year’ (yr⁻¹).

(1) (kcal m⁻²) yr⁻¹ — Correct. This is energy produced on one square metre in one year.

(2) gm⁻² yr⁻² — Incorrect. The area part (g m⁻²) is right, but time must be yr⁻¹. Yr⁻² would mean ‘per year per year’, which is not a rate of production.

(3) (kcal m⁻²) yr⁻² — Incorrect. The energy part is right, but again the time is wrongly written as yr⁻².

(4) gm⁻¹ yr⁻² — Incorrect. Both parts are wrong. Area needs m⁻² (square metre), and time needs yr⁻¹.

Note: A similar question was also asked in CUET (UG) 2025 on 20-05-2025 (Slot 1).

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.”