Think about vs Think
think about: 대상에 관련한 어떤 생각을 하다 '~에 관해 생각하다. 숙고하다.'
think of: 대상 자체를 떠올리며 생각하다 '~을 생각해 내다. 떠올려 내다.'
예문
What did they think about our proposal?
What did you think about the professor's lecture?
She couldn't think of the right words
I can't really think of the appropriate words
* What do you think of / about his new book?
대상을 어떻게 생각하느냐는 의미이므로, think of 도 맞는 표현이지만,
think about으로 대체하여도 무리가 없음.