Which one is the answer? It’s a multiple question...

Explanation:

you could include the points such as,

--> photosynthesis produces O2 which is necessary for respiration.

-->photosynthesis reduces the amount of CO2 in the atmosphere as it uses it up in the reaction. this controls the atmospheric rate of CO2 and reduced the greenhouse effect.

--> plants are the primary producers in a food chain and so, photosynthesis plays a primary role in the stability of food chains as they're the primary source of glucose as all the higher consumption is based off the producer.

--> virtually seeing, all the energy within the biosphere is based off plants that depends on photosynthesis.

--> If photosynthesis ceased, there would soon be little food or other organic matter on Earth, most organisms would disappear, and Earth's atmosphere would eventually become nearly devoid of gaseous oxygen.

there could be more but this is all I could think of.

hope it helps. :)