This is not an answer you want to wing. Ramble on, and you could turn a hiring manager off. But there are two easy steps you can take beforehand to knock this answer out of the park.
Do your research. First, read the job description and its requirements over and over. If youre familiar with the job, then you can tailor your answer to better fit the position. For example, if the job description says that the employer is looking for a person that is a self-starter and a resourceful problem solver, then saying youre motivated by the opportunity to be proactive and work independently in a position might sit well with the interviewer. Its also worth researching the companys mission and values to see if you can align your answer with them for example, if youre interviewing with a nonprofit, you might say that youre motivated by the chance to make a difference in the world.
Prepare an anecdote. Its one thing to say that youre motivated by the chance to work as a problem-solver, but another entirely to share an anecdote about how your quick thinking saved the day during a previous work crisis. When you answer what motivates you, explaining your motivation as well as providing an example of that motivation at work is a winning combination.