Is it B?

(1) Just knowing a1 doesn't help as k is unknown. If k is a big negative number, all terms might be less than 10, and if k is huge positive number, all may be greater than 10.

(2) If k is positive, a9 through a15 (7 terms) are greater than 10 and a1 through a7 (7 terms) are less than 10.
If k is negative, a9 through a15 are less than 10 and a1 through a7 are more than 10.
So in either case the answer is 7.