Option B is correct because the textbook explicitly states that 'Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR).' Options C (1-5%) and D (2-10%) are incorrect as they refer to the percentage of PAR that plants actually capture, not the percentage of incident solar radiation that constitutes PAR. The text mentions 'Plants capture only 2-10 per cent of the PAR' which is a different concept altogether. Option A (100%) is incorrect as not all solar radiation is photosynthetically active.