Hello,
The answer is option A "school".
Reason:
A social infrastructure is what something socially happens that makes the community stronger. Its option A because schools hold fundraisers, bake sells, etc... which require alot of social business and it also brings people together for a cause making the community stronger. Its not option B because not alot of people visit farms its most likely a farmers market which is where a group of framers meet therefore making a better it a better community. Its not option C because a store is where people go to get there foods they do not go there to socialize. Its also not option D because shopping malls do require socialization because of the size, color, or kind of a product you want but it is not strengthening the community in anyway. Therefore the answer is option A.
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit