The correct option is
A For
'Option A: 'For' means 'with the object or purpose of'. It relates to the given sentence here: the leaves eat carbon and release oxygen for the purpose of producing a lot of products.
Hence option A is correct.
Option B: 'In' indicates 'inclusions within a space'.
Option C: 'On' refers to the top or surface of.
Option D: 'At' indicates 'a point or place in a space'.
These words do not form a meaningful sentence.
Hence options B, C and D are incorrect.