Why are they asking?

The interviewer wants to find out the following:

  -How committed are you to the career and the company
  -How much do you understand about the career path ahead of you
  -How passionate are you

Approach 
Research the job and the organization structure and relate your career plan and progression with the job posting and progression of the company.

Eg.
I would like to work hard and grow with the company to a respectable position with additional challenges and responsibilities and contributing to success of the company.

Please don't give these answers...
"Doing your Job"
"Working somewhere else"
"No idea"
"I haven’t thought that far ahead"
"Have a family"

Most people say "Yes" and do not add more to it. Use this opportunity to market yourself and express interests and activities that are aligned with job role you are applying.