I graphed it out and it seems that both A and C are correct. I would check them to make sure.
N(N ∩ S ∩ K) = 10
n(ξ) = 250
n(S ∪ K) = 15 - 10 = 5
n(N ∪ S) = 20 - 10 = 10
n(N ∪ K) = 30 - 10 = 20
n(S) = 50 - 10 - 5 - 10 = 25
n(K) = 55 - 20 - 5 - 10 = 20
n(N) = 100 - 10 - 20 - 10 = 60
n(N ∪ S ∪ K) = 10 + 5 + 10 + 20 + 25 + 20 + 60 = 150
Therefore, n(N ∪ S ∪ K)' = 250 - 150 = 100
Therefore, 100 million people do not read any of the three papers.
3^x = 729
log(3^x) = log(729)
xlog(3) = log(729)
x = log(729)/log(3)
x = 6
