Please read the NCERT carefully. There is no mistake in the question.

NCERT: "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 (gm–2) or energy (kcal m–2)."

But in ncert gram is represented as g and not gm so here gm^-2 means to us gram metre^-2 and not gram^-2.So the correct answer as per ncert should be b only.