Greetings!
The noun (buildings) is a possessive noun, meaning that it has a relationship of owning the other noun (windows). However, the possessive noun in this case is plural (ends with s) and is thus treated differently than a singular possessive noun:
Singular:
Building's Windows
Plural:
Buildings' Windows
In this scenario, since possessive noun end in s/is plural, we don't add the s.
The answer would therefore be the last option, "buildings' windows"
Hope this helps!
-Benjamin
