Which phrase best completes the sentence: "I am confident ________ in his career"?

The choice "of his chances of succeeding" perfectly completes the sentence due to its grammatical structure and contextual clarity. The preposition "of" is typically used to indicate possession or association, making it suitable in the context of expressing confidence regarding someone's chances for success. This phrase implies a direct connection to the person's potential outcomes, essentially communicating that the speaker firmly believes in the person's likelihood of achieving success.

Moreover, the language used—"chances of succeeding"—is a common English expression that resonates with the idea of confidence in someone's future performance or achievements. It naturally conveys a sense of positive expectation regarding the person's capabilities.

In contrast, the other options struggle with either grammatical suitability or clarity of expression. "About his likelihood of success" tends to suggest a more generalized opinion rather than assured confidence. "With his prospects in business" shifts the focus to business prospects rather than a personal viewpoint on capability. Finally, "in his abilities to achieve" may imply confidence in capabilities but tends to be less direct regarding success, making it less precise in this context.