Solution: Since it is found in all photosynthetic plants except bacteria, chlorophyll an is known as the universal photosynthetic pigment. As it performs the primary photosynthesis process, which involves the conversion of light chemical energy, it is also known as primary photosynthetic pigment. Chlorophyll b and carotenoids (carotenes and xanthophylls) are considered accessory pigments because they absorb light energy of various wavelengths, extend the spectrum of light absorption, and pass the energy to chlorophyll a via electron spin resonance. So, the correct answer is ‘Chlorophyll a’.