Analysis
1. Blank (i): The Overall Quality

The word in Blank (i) must summarize the main point of the sentence. The sentence describes a contradiction: Jansen is good at handling difficult topics but struggles with easy, informal writing.
- A writing style that appears contradictory is paradoxical.
- C. paradoxical (Seemingly absurd or self-contradictory) - Correct.
2. Blank (ii): Formal Tone

Jansen is successful in enduing "the most recondite topics" (obscure, difficult) with a certain tone. Since this ability is set up to contrast with his stilted informal writing, the tone must be light, accessible, or easygoing.
- F. breezy (Casual, lively, or lighthearted) - Correct. It is surprising that he can make difficult topics feel easy and light.
3. Blank (iii): Informal Prose

The contrast is completed here. If his formal writing is surprisingly breezy, then his informal correspondence must be surprisingly awkward or stiff.
- G. curiously stilted (Stiff, unnatural, or constrained) - Correct. The irony is that his informal letters are stiff, while his academic prose is light.

Final Solution
The completed sentence is:
Jansen's writing strikes many as paradoxical (i): for one who is capable of enduing even the most recondite topics with a breezy (ii) tone, his prose becomes curiously stilted (iii) in the informal correspondence he had with his contemporaries.

Blank (i): C. paradoxical
Blank (ii): F. breezy
Blank (iii): G. curiously stilted