Select the option that can be used as a one-word substitute for the underlined words.
He was promoted to a higher position in the first year due to his careful nature and hard-working efforts.

The correct option is diligence.

Let's analyze the meaning of the underlined phrase and the given options to find the correct one-word substitute:
The underlined phrase is "careful nature and hard-working efforts".

Let's look at the definitions of the options provided:
1. business mindedness: The quality of being interested in and good at business; focusing on commercial opportunities.
2. extraordinary: Very unusual, remarkable, or exceptional.
3. elegance: The quality of being graceful and stylish in appearance or manner.
4. diligence: Careful, persistent, and hard-working effort.

Since the word "diligence" directly represents a careful nature combined with persistent, hard-working efforts, it is the perfect one-word substitute for the underlined phrase in the sentence.