Answer: The correct answer is-
Stoma allows the gaseous reactants and products of photosynthesis to move in and out.
Stoma ( plural stomata) are the tiny holes/ pores that are primarily found on the surface of leaf ( sometimes also presnt in stem) and help in the exchange of gases ( such as carbondioxide and oxygen, which are the gaseous reactants and products of photosynthesis respectively). They are surrounded by guard cells that regulate the opening and closing of stomata.