for large values of k, A will take more time then B
A>>>>B (for large k)
so, for large k, B always prefer
but in question they are asking what is the smallest value of k that B prefer
let take for k=5, A-> 106 and B->5*106 (A<B) so A will prefer
for k=6, A-> 108 and B->6*107 (A>B) so B will prefer
(C.) is the correct option.