He was a (ambition?) person in spite of his great success.

He was a (ambition?) person in spite of his great success.
He was a () person in spite of his great success.
选项:
a、a. ambition
b、 b. modest
c、 c. observe
d、 d. application
搞不清楚答案为什么是这个呀，能不能提醒一下？