It's (c). "Жилых строений:" is a label and "1" is the exact quantity. Yes, genitive plural is normally used for such labelsthe noun. However, the numeral is in the nominative, agreeing with the noun in gender and number, e.g.
- Ножей: 2 (два)
- Вилок: 2 (две)
- Ложек: 1 (одна)
- Ножниц: 1 (одни)
Ножницы (scissors) are pluralia tantum and thus один has to be plural as well (одни). Remarkable, isn't it? Russian has plural for the numeral 'one'.