C, I think..... with B alone, from 0 to 27 can be answers...
Whatever be the case the number will be 27. remember it is asking how many have swimming pool not how many have only swimming pool.
Let number of houses having both pool and patio be x hence number of people having neither is x
Let number of people having only pool be y
75 = 48 + (y+x) +x(neither) -x (both)
y+x (pool) = 75-48 = 27