The correct option is C windows
There are two numbers in Noun-Number: Singular and Plural. When we speak about one person / thing, we use the noun in singular form. When we speak about more than one person / thing, we use the noun in plural form.
Since, in the given sentence, we are referring to two separate buildings, we need to use a plural noun. The only plural option is option B) windows. Hence, this is the right answer.