Breakdown of the Blank

The sentence sets up a comparison and contrast between two writers: Mikhail and Dimitri.
- Similarity: Both had equal talent (Mikhail perhaps more gifted).
- Contrast/Result: Dimitri thrived by relying on his "skills of self-promotion." Mikhail, however, "switch[ed] careers."

Mikhail's action (switching careers, giving up) contrasts with Dimitri's successful self-promotion. Therefore, Mikhail's characteristic must be one that hindered his ability to compete and promote himself. It must be a lack of self-confidence or a reluctance to put himself forward.
- A. naivete: Lack of experience, wisdom, or judgment. While possible, it doesn't directly explain a failure to self-promote.
- B. persistence: Firm continuance in a course of action. This is the opposite of switching careers.
- C. diffidence: Modesty or shyness resulting from a lack of self-confidence. This is a strong fit, as a lack of self-confidence would prevent effective self-promotion in a competitive market.
- D. complacency: A feeling of uncritical satisfaction with oneself or one's achievements. This might lead him to switch if he didn't care about success, but diffidence creates a better contrast with Dimitri's active promotion.
- E. pride: A feeling of deep pleasure or satisfaction derived from one's own achievements. This is the opposite of the needed trait.
- F. timidity: Lack of courage or confidence; easily frightened. This is a very close synonym to diffidence and explains why he would fail to self-promote and ultimately give up his career.

The two words that mean lack of self-confidence or shyness, which would hinder career success in a competitive market, are diffidence and timidity.

The correct choices are $\(\mathbf{C}\)$. diffidence and $\(\mathbf{F}\)$. timidity.