A 2.6.9-ac1 kernel would be

A.
an alpha kernel.

B.
someone’s patch to a stable kernel.

C.
a stable kernel.

D.
an unstable kernel.

E.
someone’s patch to an unstable kernel.

Explanation:
2.x is a stable kernel if x is an even number. Kernels are numbered like 2.6.37. (see www.kernel.org)