Which of the following statements best describes "carbon fertilization"?

1. A

   Increased plant growth due to increased concentration of carbon dioxide in the atmosphere

   (a) Increased plant growth due to increased concentration of carbon dioxide. Extra atmospheric CO2 can raise photosynthetic rates and biomass where water and nutrients allow—this is carbon (or CO2) fertilisation. That is the definition asked. This option is the official key.

2. B

   Increased temperature of Earth due to increased concentration of carbon dioxide in the atmosphere

   (b) Increased temperature of Earth due to increased CO2. That is the greenhouse / global-warming effect, not the fertilisation metaphor. This option is not the key.

3. C

   Increased acidity of oceans as a result of increased concentration of carbon dioxide in the atmosphere

   (c) Increased acidity of oceans from extra CO2. That is ocean acidification (lower pH, carbonate chemistry), a different CO2 impact. This option is not the key.

4. D

   Adaptation of all living beings on Earth to the climate change brought about by the increased concentration of carbon dioxide in the atmosphere

   (d) Adaptation of all living beings to climate change from extra CO2. That overstates both adaptation and the meaning of the term. This option is not the key.

Summary. Official key is (a). Carbon fertilisation means plants, as a class, can grow more when the air is richer in CO2, the substrate of photosynthesis. Greenhouse warming, ocean acidification, and a blanket “all beings adapt” story are real or rhetorical neighbours, not that term. The stored letter therefore names increased plant growth. Options (b), (c) and (d) describe other CO2 pathways.