請問小白兔等大俠,H1B transfer

I am working on H1B visa which will expires in Oct this year. I got another offer and the employer would like to transfer my H1b to the new employer. And I want to extend for another three(3) years. For the part 2 of I-129-Basis for Classification, which should I choose? (a)New employment (including new employer filing H-1B extension). (b)Change of employer.Thanks!