The process that photoautotrophs use to produce their food is photosynthesis.
So, option d ( photosynthesis) is correct .
Photosynthesis is the process which is used by the plants . In which plants uses sunlight energy and convert carbon dioxide ,water into the molecules which is required for growth. Some of the molecules are sugar, enzymes etc.
The reaction of photosynthesis is :
6CO2 + 6H2O ------> C6H12O6 + 6O2.