The approximate ratio of chlorophyll a to chlorophyll b in most plants is 3:1. In other words, plant tissues commonly contain about three chlorophyll a molecules for every one chlorophyll b molecule, although the exact ratio varies.
Chlorophyll a is the primary photosynthetic pigment in oxygen-producing photosynthesis. It participates directly in the reaction centers that convert light energy into chemical energy. Chlorophyll b is an accessory pigment: it absorbs somewhat different wavelengths and transfers the captured energy to chlorophyll a, broadening the light that a plant can use.
The 3:1 figure is a useful generalization, not a fixed rule for every leaf. Plants adjust their pigment composition according to light intensity, shade, species, leaf age, and nutritional conditions. Shade-adapted leaves often have relatively more chlorophyll b, helping them harvest limited light. Sun-grown leaves frequently have a higher chlorophyll-a-to-b ratio.
Another common mistake is reversing the ratio. A 1:3 ratio describes chlorophyll b to chlorophyll a when the same approximate relationship is expressed in the opposite direction. The quiz asks specifically for a-to-b, so 3:1 is the canonical answer.